Sara Ibrahim
email: sara.ibrahim@univ-orleans.fr

Fatima Zahrae Lahboub
email: fatima-zahrae.lahboub@etu.univ-orleans.fr

Pascal Brault
email: pascal.brault@univ-orleans.fr

Agnès Petit
email: agnes.petit@univ-orleans.fr

Amael Caillard
email: amael.caillard@univ-orleans.fr

Eric Millon
email: eric.millon@univ-orleans.fr

Thierry Sauvage
email: thierry.sauvage@cnrs-orleans.fr

Asunción Fernández
email: asuncion@icmse.csis.es

Anne-Lise Thomann
email: anne-lise.thomann@univ-orleans.fr

Amaël Caillard

Influence of helium incorporation on growth process and properties of aluminum thin films deposited by DC magnetron sputtering

Keywords: Nanostructured aluminum films, porous thin films, direct current magnetron sputtering, heliumargon mixture plasma

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Introduction

The study of helium (He) behavior in metals has drawn significant attention in the materials science community for several years. He is produced in charged tritium storage media and is inserted in the structural materials of fission and fusion reactors [START_REF] Zheng | Introduction of helium into metals by magnetron sputtering deposition method[END_REF]. The accumulation of He in these materials could lead to the degradation of their mechanical properties [START_REF] Shi | Helium-charged titanium films deposited by direct current magnetron sputtering[END_REF]. Hence, the mechanism of He diffusion in the metal lattice is of great interest in nuclear technology, and it has been widely studied [START_REF] Chamssedine | DIADDHEM set-up: New IBA facility for studying the helium behavior in nuclear glasses[END_REF][START_REF] Pentecoste | Low Energy and low fluence helium implantations in tungsten: Molecular dynamics simulations and experiments[END_REF]. Due to its insolubility in metals, helium can diffuse on a long pathway and induce the formation of vacancy defects by a self-trapping mechanism, even at kinetic energies below the metal displacement threshold [START_REF] Pentecoste | Substrate temperature and ion kinetic energy effects on first steps of He + implantation in tungsten: Experiments and simulations[END_REF][START_REF] Tanyeli | Nanostructuring of iron surfaces by low-energy helium ions[END_REF]. These He-vacancy complexes can act as trapping sites for other incoming He atoms until large bubbles are stabilized [START_REF] Wang | Evolution behavior of helium bubbles and thermal desorption study in helium-charged tungsten film[END_REF]. In certain conditions, when these processes take place close to the surface and are associated with metal atoms' mobility, flaking and nanofibers have been observed [START_REF] Tanyeli | Nanostructuring of iron surfaces by low-energy helium ions[END_REF][START_REF] Fan | Efficient plasma-assisted approach in nanostructure fabrication of tungsten[END_REF][START_REF] Iyyakkunnel | Morphological changes of tungsten surfaces by low-flux helium plasma treatment and helium incorporation via magnetron sputtering[END_REF][START_REF] Takamura | Formation of nanostructured tungsten with arborescent shape due to helium plasma irradiation[END_REF]. Thus, low-energy He plasma irradiation allowed synthesizing nanostructured and porous materials with tailored functionalities. These nanostructures modify the material properties and can be relevant for different applications [START_REF] Kajita | Development of nanostructured black metal by self-growing helium bubbles for optical application[END_REF][START_REF] Kajita | Helium plasma implantation on metals: Nanostructure formation and visiblelight photocatalytic response[END_REF][START_REF] Giarratano | Nanoporous Pt-based catalysts prepared by chemical dealloying of magnetronsputtered Pt-Cu thin films for the catalytic combustion of hydrogen[END_REF]. For instance, Fan et al. reported the formation process of the fiberform nanostructured (fuzz) tungsten layer by He plasma irradiation [START_REF] Fan | Efficient plasma-assisted approach in nanostructure fabrication of tungsten[END_REF]. The formation of metallic nanostructures by exposure to He plasmas has been also evidenced in other metals such as nickel, iron and aluminum [START_REF] Kajita | Helium plasma implantation on metals: Nanostructure formation and visiblelight photocatalytic response[END_REF][START_REF] Tanyeli | Surface modifications induced by high fluxes of low energy helium ions[END_REF].

Recently, magnetron sputtering has been employed as an efficient method to introduce 4 He and 3 He atoms and induce porosity formation into thin films [START_REF] Godinho | On the formation of the porous structure in nanostructured a-Si coatings deposited by dc magnetron sputtering at oblique angles[END_REF][START_REF] Lacroix | The nanostructure of porous cobalt coatings deposited by magnetron sputtering in helium atmosphere[END_REF][START_REF] Schierholz | STEM-EELS analysis reveals stable high-density He in nanopores of amorphous silicon coatings deposited by magnetron sputtering[END_REF][START_REF] Fernández | Low gas consumption fabrication of 3He solid targets for nuclear reactions[END_REF]. Zheng et al. [START_REF] Zheng | Introduction of helium into metals by magnetron sputtering deposition method[END_REF] and Shi et al. [START_REF] Shi | Helium-charged titanium films deposited by direct current magnetron sputtering[END_REF] obtained titanium films containing a high concentration of He by magnetron sputtering with He/Ar mixture. It has been shown that He incorporation may lead to the formation of nanosized He filled bubbles or porous thin films depending on the material and the process parameters [START_REF] Godinho | On the formation of the porous structure in nanostructured a-Si coatings deposited by dc magnetron sputtering at oblique angles[END_REF][START_REF] Lacroix | The nanostructure of porous cobalt coatings deposited by magnetron sputtering in helium atmosphere[END_REF][START_REF] Giarratano | Nanoporous Pt-based catalysts prepared by chemical dealloying of magnetronsputtered Pt-Cu thin films for the catalytic combustion of hydrogen[END_REF].

This work aims at determining the optimal deposition parameters to produce Al films with the highest open porosity/specific surface area for hydrothermal hydrogen production [START_REF] Bergthorson | Recyclable metal fuels for clean and compact zero-carbon power[END_REF]. The other objective is to get better insight into the mechanism involved in the growth process. We investigated the microstructure and composition of Al thin films prepared by direct current (DC) magnetron sputtering in different He/Ar mixed atmospheres. For this purpose, different deposited films were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Moreover, Rutherford backscattering spectrometry (RBS) and proton elastic backscattering spectrometry (PEBS) were carried out to determine the number of Al and He atoms in the films, respectively. Gas-phase characterization and simulations including mass spectrometry, SRIM software and collisional model have completed the study towards understanding the film's growing mechanism.

Materials and methods

Deposition experiments

Al films were deposited onto (100) oriented p-doped Si wafers by sputtering a 4-inch Al target (99.999% purity from Neyco). The substrates were pasted on a rotating substrate holder, and the distance between the substrate and the target was fixed at 12 cm. The system was kept under vacuum, reaching a base pressure of about 7 x 10 -4 Pa. The discharge current was regulated to 1 A using a DC power supply (Pinnacle plus from Advanced Energy). Then, depending on the process parameters, the cathode voltage varied between 325 and 440 V, resulting in a power range of 325-440 W. The gas flow rates were adjusted using two Bronkhorst EL-FLOW mass flow controllers. The argon and helium flow rates (ՓAr and ՓHe) were varied to obtain a He percentage (ΓHe = ՓHe / (ՓAr + ՓHe) x 100) ranging between 0 and 100%. The total pressure in the chamber was settled to 1 Pa using a gate valve before turning on the discharge.

It is expected that the sputtering yield of metal atoms by He ions is much lower than that by Ar ones [START_REF] Liu | Investigations of helium incorporated into a film deposited by magnetron sputtering[END_REF]. From Stopping and Range of Ions in Matter (SRIM) simulation [START_REF] Ziegler | SRIM-The stopping and range of ions in matter[END_REF], the sputtering yield of Al by 400 eV He + ions (0.29 atoms/ion) is determined to be twice less than that by 400 eV Ar + ions (0.58 atoms/ion). The deposition time (td) was chosen to deposit Al films of ⁓1 µm thickness (as determined from SEM measurements) in different He/Ar mixtures, as shown in Table 1. To estimate the reproducibility of the experiments, some films were deposited twice and characterized.

Gas phase characterization

SRIM software had been used to determine the nature and energy of particles emitted at the target under bombardment with 400 eV Ar or He ions. Starting from these data, a simple collisional model of particle slowing down along straight-line trajectories and subjected to a continuous energy loss process, has been implemented to determine the energy distribution functions (EDF) of the particle flux in pure argon or helium gas as a function of the distance from the source [START_REF] Xie | Molecular dynamics simulations of clusters and thin film growth in the context of plasma sputtering deposition[END_REF]. Moreover, the mean kinetic energy of the sputtered atoms was calculated according to a modification of the Thompson atomic model [START_REF] Xie | Molecular dynamics simulations of clusters and thin film growth in the context of plasma sputtering deposition[END_REF]. The sputtering gas temperature was fixed at 300 K with a pressure and a target-substrate distance values identical to the deposition conditions. Additional information about the model can be found elsewhere [START_REF] Xie | Molecular dynamics simulations of clusters and thin film growth in the context of plasma sputtering deposition[END_REF]. In addition, the influence of ΓHe on the ionization of Al, Ar and He was determined by measuring their ion energy distribution functions (IEDF) using a EQP1000 energy-resolved mass spectrometer (Hiden Analytical) between 1 and 20 eV in 0.1 eV/charge steps. The spectrometer was placed on the axis of the Al target with a distance of 8 cm from the electrically grounded 200 µm in diameter orifice. An auto-tune procedure has been performed in order to adjust the different internal parameters of the mass spectrometer and to optimize the signal intensity. A power of 200 W and a pressure of 1 Pa were used during this experiment.

Morphological and structural characterizations of films

The surface and cross-sectional morphology of the deposited films were investigated by scanning electron microscopy (Carl Zeiss SMT, Supra-40, FEG-SEM). The accelerating voltage in the range of 3-4 kV has been used. The evolution of the crystalline structure of Al films with different He concentrations was characterized using a Bruker D8 Discover X-ray diffractometer in the Grazing incidence geometry (GIXRD). In addition, the crystallite size (D) was determined from diffractograms obtained in the Bragg-Brentano configuration. The diffractometer is equipped with a Cu Kα radiation (λ = 0.154 nm) source, where 2ϴ was varied from 35 to 90° with 0.02°/step.

Composition of films

The quantity of Al in the films was determined by means of RBS with 2 MeV alpha particles.

Moreover, the amount of He in the films was investigated by PEBS using 2.4 MeV protons.

The Ion beam analysis (IBA) experiments were carried out at Pelletron accelerator facility of CEMHTI laboratory (Orléans), a facility that is part of the EMIR&A French network of accelerators (http://emir.in2p3.fr). A 25 mm 2 Passivated Implanted Planar Silicon (PIPS)

detector is located at 166° scattering angle and 90 mm distance. The experimental spectra were simulated using the SIMNRA software [START_REF] Mayer | Improved physics in SIMNRA 7, 21 st[END_REF] and the differential cross-sections for spectra simulation were determined from the SigmaCalc Internet site [START_REF] Gurbich | SigmaCalc recent development and present status of the evaluated crosssections for IBA[END_REF]. The proportion of He incorporated inside the films was determined as the ratio of the areal atomic density of He atoms (N He PEBS ) to that of Al atoms (N Al RBS ). The uncertainty on NAl was evaluated to ± 5% considering the statistical error on the Al RBS signal. Then, the uncertainty on He proportion was estimated to ± 15% taking into account the statistical error on both He PEBS and Al RBS signals.

Results

Plasma and sputtering process

Figure 1 shows the variation of the cathode voltage (V) as a function of the percentage of He in the gas phase. Three main zones could be identified. For zone A (ΓHe ˂ 70%), the voltage remains approximately constant near 370 V. So, in this region, introducing He does not significantly modify the plasma. This is not surprising since He has a smaller ionization cross section (σHe = 4.25 x 10 -21 m 2 ) than that of Ar (σAr = 3.25 x 10 -20 m 2 ) [START_REF] Abdel-Kader | Characterization of the electrical breakdown for DC discharge in Ar-He gas mixture[END_REF].

In zone B, the absolute value of the voltage decreases from 370 to 320 V as the percentage of He increases from 70 to 93%, indicating that He starts influencing the plasma features. The observed trend at a fixed current suggests a rise of the ion flux at the cathode, revealing an increase in the electron and ion density. Several authors have studied the change in plasma characteristics when another inert gas is added to Ar in sputtering plasma discharges [START_REF] West | Influence of inert gas species on the growth of silver and molybdenum films via a magnetron discharge[END_REF][START_REF] Aijaz | A strategy for increased carbon ionization in magnetron sputtering discharges[END_REF].

The primary reported changes were attributed to the difference of the ionization potential. It is known that the ionization energy of the gas atom plays a major role in different mechanisms that influence the ionization degree in the plasma [START_REF] Cuynet | An efficient way to evidence and to measure the metal ions fraction in high power impulse magnetron sputtering (HiPIMS) postdischarge with Pt, Au, Pd and mixed targets[END_REF]. Indeed, this parameter determines the secondary electron emission coefficient of the ions impacting the target [START_REF] Gudmundsson | An ionization region model of the reactive Ar/O2 high power impulse magnetron sputtering discharge[END_REF], the electron impact ionization cross section of the gas atoms, and indirectly the electron energy distribution function. For instance, it has been shown that substituting argon (15.75 eV) by gases of higher ionization energies, like He (24.59 eV) [START_REF] Abdel-Kader | Characterization of the electrical breakdown for DC discharge in Ar-He gas mixture[END_REF] or Ne (21.6 eV) [START_REF] Livengood | The neon gas field ion source-a first characterization of neon nanomachining properties[END_REF], induces an increase of the electronic temperature and thus the high energy electron contribution in the energy distribution function [START_REF] Sahu | Effect of helium incorporation on plasma parameters and characteristic properties of hydrogen free carbon films deposited using DC magnetron sputtering[END_REF]. This leads to a rise in the ionization degree of other species (Ar and sputtered Al atoms) in the plasma. A high ionization potential promotes the secondary electron emission from the target which sustains the discharge and increases the electron and ion density.

Moreover, in the particular case of Ar/He mixture, other mechanisms like asymmetric charge transfer and metastable-neutral ionization have been found to increase the ionization degree [START_REF] Schucan | Niobium films produced by magnetron sputtering using an Ar-He mixture as discharge gas[END_REF]. These observations are in agreement with the decrease in the cathode voltage absolute value observed in zone B.

In zone C, the voltage increases sharply, reaching 440 V for 100% He. This is the result of a transition from Ar to He-dominated plasma. At this step, the main partner of electron/neutral collisions becomes He, which leads to a less efficient ionization process and, thus, to a rapid decrease of the electron density. Again, to maintain the current constant, the cathode voltage has to increase in absolute value. Similar high values of the cathode voltage in absolute value have been reported in pure He or Ne sputtering discharges by several authors [START_REF] West | Influence of inert gas species on the growth of silver and molybdenum films via a magnetron discharge[END_REF][START_REF] Kusano | Influence of inert gases on ionized magnetron plasma deposition of carbon nitride thin films[END_REF]. It was also reported that pure helium discharges were challenging to initiate due to the low ionization cross-section of He [START_REF] Abdel-Kader | Characterization of the electrical breakdown for DC discharge in Ar-He gas mixture[END_REF][START_REF] Kusano | Influence of inert gases on ionized magnetron plasma deposition of carbon nitride thin films[END_REF]. Therefore, ignition was maintained by increasing the pressure, pulsing the target power, or adding a small percentage of Ar gas. The sharp rise of the cathode voltage shown in Figure 1 indicates that the plasma regime is completely modified above 93 % of He in the gas phase, which is expected to influence the sputtering process and the film growth to a great extent.

The deposition rate of Al (νAl) has been determined from RBS measurements, which is the number of Al atoms deposited per unit of area and time. Figure 2 presents the variation of νAl as a function of the ΓHe in the gas phase. First, it is observed that the deposition rate remains high and approximately constant until 93% of He. Then, beyond 93%, the deposition rate decreases because He ions start contributing significantly to the sputtering of Al.

Energy distribution functions (EDF)

Figure 3 shows the energy distribution functions at the substrate position of Al atoms sputtered from the target by pure Ar + or He + ions. It is clearly shown that in pure He plasma, Al atoms will arrive at the substrate with a higher mean kinetic energy (2.7 eV compared to 0.16 eV in pure Ar). These results were expected and well known. For instance, Schuan et al. used a mixture of He and Ar to promote the energy of the depositing atoms and enhance Nb film quality [START_REF] Schucan | Niobium films produced by magnetron sputtering using an Ar-He mixture as discharge gas[END_REF]. The higher energy of Al atoms in case of He plasma could be explained by the low mass of He which reduces the energy loss by He atoms colliding with the sputtered gas atoms [START_REF] Schucan | Niobium films produced by magnetron sputtering using an Ar-He mixture as discharge gas[END_REF]. One has to keep in mind that besides these trends of the energy distribution function, shifting from Ar + to He + also reduces the number of sputtered Al atoms.

Another significant result is obtained from SRIM simulations showing that a non-negligible part of high energy He + ions (17%) is backscattered at the target. These ions become fast neutrals that interact with the substrate during the film growth. Considering the respective angular distributions of sputtered Al atoms and backscattered He + ions, a rough estimation yields that one fast He neutral arrives at the substrate for ten depositing Al atoms. Since the He + ions have been accelerated through the cathode sheath (400-440 eV), they exhibit a relatively high mean kinetic energy: 145 eV at the target and 60 eV 12 cm away at the substrate position, which is above the sputtering threshold of aluminum under He ion bombardment [START_REF] Yamamura | Energy dependence of ion-induced sputtering yields from monatomic solids at normal incidence[END_REF]. On the contrary, SRIM simulations predicted that the probability of Ar ions to be backscattered is negligible (less than 0.2%) in our experimental conditions.

Figure 4 displays the IEDF of Al + , Ar + and He + ions between 1 and 20 eV obtained from the different gas mixtures. The ion energy distribution functions of Al + ions in different Ar/He gas mixtures exhibits an intense peak at low energy (below 1 eV) corresponding to the almost thermalized ionized sputtered species. This intense signal was not registered during the energyresolved mass spectrometer measurements. Figure 4a shows the presence of energetic ionized population of sputtered aluminum usually observed for metals in DC magnetron discharges [START_REF] Aijaz | A strategy for increased carbon ionization in magnetron sputtering discharges[END_REF][START_REF] Andersson | Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar/O2 mixtures[END_REF][START_REF] Gudmundsson | Physics and technology of magnetron sputtering discharges[END_REF]. As ΓHe increases above 70%, the high energy tail is promoted showing that sputtered Al atoms lose less energy by collision during their transport towards the substrate. This is in good agreement with SRIM data obtained in pure Ar and He gas. Significant changes are observed in the energy distribution function of Ar + and He + at high ΓHe: the peak below 5 eV splits into two contributions and the high energy tail (between 5 eV and 10 eV) is enhanced.

These trends are promoted as the He percentage increases in the gas phase. A complete understanding of the features of these energy distribution functions is beyond the scope of this paper. However, the impact of He metastable atoms present at high He partial pressure may be suspected. Nevertheless, regarding Al thin film growth, what has to be retained is the increase of the energy brought by the particles (sputtered Al, Ar + , He + , fast neutrals and metastable atoms) interacting with the substrate.

Thin film characteristics

Figure 5 presents cross-section and top-view SEM images of selected Al films obtained with different ΓHe in the gas phase. It is evident that the composition of the gas phase significantly influences film morphology. Films deposited with ΓHe lower than 70% are composed of large 3D faceted grains, as shown in Figure 5 (1,b) and Figure 5 (2,b). Even if a zone at the interface with the substrate seems to exhibit small pores, a very dense structure is observed over 2/3 of the thickness. The size of the grains is in the order of 200-300 nm. In the so-called structure zone model, this kind of morphology is usually observed in zone T, when the surface mobility of the depositing atoms is sufficient to induce initial grain coarsening [START_REF] Petrov | Microstructural evolution during film growth[END_REF][START_REF] Mahieu | Biaxial alignment in sputter deposited thin films[END_REF][START_REF] Anders | A structure zone diagram including plasma-based deposition and ion etching[END_REF]. As ΓHe increases up to about 90%, the grain size observed in top-view image decreases down to less than 100 nm. A quite dense columnar microstructure is visible on the cross-section images even if small voids exist between columns, as shown in Figure 5 (3,a). This columnar structure is typically obtained in the case of magnetron sputtered films deposited under limited mobility of adatoms such that the film growth is driven by surface shadowing [START_REF] Petrov | Microstructural evolution during film growth[END_REF]. The morphology of the 95% sample appears quite different from the previous ones. Clearly, two families of grains are visible, exhibiting different diameters and heights and surrounded by immense voids. The surface roughness is expected to be high, and the typical grain size ranges from 100 to 200 nm and 200 to 400 nm, as shown in Figure 5 (4,c). 50 nm pores are also present and distributed homogeneously along with the film thickness (Figure 5 (4,b)). Thereafter, a different morphology is obtained with pure He plasma. In this case, the film is completely porous, and it is composed of fiberform nanostructures (⁓ 100 nm in diameter) that seem to be grown perpendicular to the substrate surface and nanoparticles (10-20 nm in diameter), as shown in Figure 5 (5,a,b). The presence of these two microstructures in this film could result from two different Al deposition processes that will be discussed later.

The proportion of He to Al in the films, determined by RBS and PEBS measurements is shown in Figure 6. Helium is not detected in the films synthesized in He/Ar mixtures containing less than 20% of He. Moreover, He proportion remains very low (less than 5%) until 70% of He is added in the gas phase. Then, the proportion of He starts to increase, reaching a maximum of ⁓ 15% for the film deposited with 95% of He in the gas phase. The film deposited in pure He plasma, presenting the most porous sample, contains a limited amount of He (below 4%). All interpretations may explain such result. First, the film is less crystallized under He incorporation. Second, the film becomes effectively more porous. Also, it can be clearly noticed that the peak broadens and shifts to lower angle values when the percentage of He in the gas phase increases from 9 to about 70%. After that, from 91 to 100% He, different evolution is observed: the peak shifts back to its initial value, whereas the width remains constant.

To get better insight into the peak width evolution, the crystallite size (D) of the different samples was determined from the width of the -2 (111) peak, based on the Scherrer formula [START_REF]The Scherrer Formula for X-Ray Particle Size Determination[END_REF]. Despite the different deposition times, the thickness of the films was not the same. Since the crystallite size depends on film thickness [START_REF] Petrov | Microstructural evolution during film growth[END_REF], the crystallite size for an equivalent thickness of 1 µm was calculated. Figure 8 shows the evolution of this crystallite size. Again the same three zones can be identified on the graph. When ΓHe increases from 9 to 70%, the crystallite size of Al film decreases sharply from ⁓ 100 nm to ⁓ 30 nm. Between 70 and 93% He, the crystallite size is around 20 nm. Beyond 93%, a slight rise is visible, but the crystallite remains smaller than that in pure argon.

Discussion

The study of the deposition of Al films using different He/Ar gas mixtures allowed identifying three plasma regimes, as shown in Figure 1. The non-linear transition from Ar to He driven plasma has been found to modify the sputtering efficiency and the deposition conditions.

Besides, the nature of the gas may impact the species ejected from the target and their energy distribution function at the substrate [START_REF] West | Influence of inert gas species on the growth of silver and molybdenum films via a magnetron discharge[END_REF] (Figure 3). SRIM simulations predict the backscattering of a large number of high energy He + ions at the target that become fast neutrals and interact with the substrate during the film growth.

First, in zone A (ΓHe = 0-70%), the plasma is dominated by argon. Ar + ions are mainly present and induce the sputtering of the target atoms. Hence, the sputtering yield of Al is high, and the deposition rate remains high and constant in this range. The films show the dense columnar structure, as revealed by SEM images. Large and well crystallized Al grains are obtained. All these results indicate that the growth mode is not drastically modified in this range of He/Ar mixtures compared to pure Ar. Nevertheless, the size of the crystal domains decreases with increasing ΓHe (see Figure 8). Also, peak broadening from XRD analysis confirms a reduction in the crystallite size. Since the amount of He detected inside the film is very low and constant, this phenomenon could not be related to He trapping. Moreover, since ΓHe increases in the gas phase, the probability that a sputtered Al atom encounters collisions with a He atom rather than an Ar atom is promoted. Results obtained by SRIM show that this will lead to a rise in the mean kinetic energy carried by the depositing Al atoms. This trend is contradictory to the observed diminution of the crystallite size. However, another effect that could be invoked, when He is added in the gas phase, is the efficient cooling process due to the well-known high thermal conductivity of He [START_REF] Murphy | Transport coefficients of helium and argon-helium plasmas[END_REF]. The interaction of more He atoms with the substrate during the growth could reduce adatom mobility. Such a mechanism could also promote stress inside the film, which is in good agreement with the XRD peak shift observed (see Figure 7b).

In zone B (ΓHe = 70-95%), the addition of He in the gas mixture induces an increase of He atoms incorporated in the films by a factor of 3, affecting the growth. This is visible on the microstructure of the films that strongly varies in this domain (see Figure 5). From discharge parameters (Figure 1) and SRIM results, we can assume that helium ions begin to be formed and that more fast He neutrals and metastable atoms now reach the substrate. It has been previously proved that backscattered neutrals are responsible for the insertion of inert gas in magnetron sputtered thin films [START_REF] Hoffman | Compressive stress and inert gas in Mo films sputtered from a cylindrical-post magnetron with Ne, Ar, Kr, and Xe[END_REF]. The increase of trapped He in the plasma regime where He ions are supposed to be formed agrees with this statement. The presence of pores on SEM images and the fact that the crystallite size does not vary in this zone reveal that He does not perturb the growth of Al grains but is instead trapped outside the Al lattice, forming bubbles at the grain boundaries. This is in agreement with what is widely known about He behavior in metals. Because He has no affinity with metals and diffuses easily, it tends to be retained in defects becoming efficient trapping sites, leading to the growth of He-vacancy complexes and then to bubbles [START_REF] Shi | Helium-charged titanium films deposited by direct current magnetron sputtering[END_REF][START_REF] Liu | Investigations of helium incorporated into a film deposited by magnetron sputtering[END_REF].

Third, in zone C, especially in pure He, the growth regime is completely altered. This is clearly visible on the SEM images (Figure 5 (5,a,b,c)), where fiberform nanostructures coated by nanoparticles are obtained. To our knowledge, this kind of morphology is very unusual and has never been reported for Al thin films grown by magnetron sputtering. Since it has only been observed on the 100% He sample, it can be concluded that a significant change of the growth mode due to the modified plasma conditions and energy flux is certainly achieved in pure He plasma. Helium is no more efficiently inserted in the film or is desorbed during the growth, giving rise to the porous structure. A comparison with He implanted metal surfaces means that the release of the trapped He into bubbles enhances the formation of such fiber-like structure [START_REF] Kikuchi | Porous tungsten nanostructure formation using a helium arc discharge plasma under subatmospheric pressure[END_REF][START_REF] Liu | Irradiation effects of hydrogen and helium plasma on different grade tungsten materials[END_REF]. Tanyeli et al. published a comprehensive study on the surface modification of Ti, Cu, and Al after exposure to low energy He ions (˂ 100 eV) [START_REF] Tanyeli | Surface modifications induced by high fluxes of low energy helium ions[END_REF]. Different surface modifications were detected among these metals. Ti did not show significant surface modification, attributed to its closely packed structure and low sputtering yield [START_REF] Tanyeli | Surface modifications induced by high fluxes of low energy helium ions[END_REF]. However, in the case of Cu and Al, nanopillars have been obtained [START_REF] Tanyeli | Surface modifications induced by high fluxes of low energy helium ions[END_REF]. The authors explain that such patterns are formed when sputtering the surface and/or emptying of superficial bubbles are associated with a high atom mobility. Al is a perfect candidate for creating the fuzz structure due to its high sputtering yield by He + ions and low fusion temperature, ensuring high adatom mobility [START_REF] Tanyeli | Surface modifications induced by high fluxes of low energy helium ions[END_REF]. In our work, this fiber-like nanostructure is formed when favorable deposition conditions are met, which means when the ratio of fast He neutrals to the depositing Al atoms and when the energy transferred by all the particle impacting the substrate (sputtered Al, metastable He atoms, Ar + and He + ) are high. Then, the surface diffusion of deposited Al atoms is promoted.

As mentioned above, in pure He plasma, nanoparticles are also detected in the films. It is known that nanoparticles can be formed in the gas phase in magnetron sputtering, especially at high pressure, when collisions are enhanced [START_REF] Xie | Molecular dynamics simulations of clusters and thin film growth in the context of plasma sputtering deposition[END_REF][START_REF] Peter | Metal/polymer nanocomposite thin films prepared by plasma polymerization and high pressure magnetron sputtering[END_REF]. This trend is well known and used in gas aggregation source devices [START_REF] Binns | Nanoclusters deposited on surfaces[END_REF][START_REF] Ganeva | Mass Spectrometric Investigations of Nano-Size Cluster Ions Produced by High Pressure Magnetron Sputtering[END_REF][START_REF] Caillard | PdPt catalyst synthesized using a gas aggregation source and magnetron sputtering for fuel cell electrodes[END_REF][START_REF] Brault | Molecular dynamics simulations of initial Pd and PdO nanocluster growth in a magnetron gas aggregation source[END_REF]. He gas is usually employed in such experiments, again because of its cooling properties, to promote the condensation process [START_REF] Kikuchi | Porous tungsten nanostructure formation using a helium arc discharge plasma under subatmospheric pressure[END_REF]. We think that for high percentages of He in the gas phase, nucleation and growth of nanoparticles could occur in our experimental conditions.

Conclusions

In this study, we investigated the influence of He introduction in the gas phase during magnetron sputtering deposition of aluminum thin films to synthesize porous nanostructures. The addition of He in an Ar plasma affects the sputtering efficiency of the target, the transport of ejected metal atoms, the kinetics of the ionized species and the deposition conditions at the substrate.

In a first wide range of ΓHe (below 70%), we found out that a cooling process by He gas could explain the decrease of the Al crystallite size, which is the main detected modification of Al film properties. A residual amount of He is inserted inside the films (trapped gas) that does not seem to affect the growth. Then, when He starts to be ionized in the plasma, the concentration of He atoms inserted in the film increases. It is mainly located in nanoscale bubbles, such that its presence significantly modifies the microstructure. This behavior is not far from what is observed during He ion implantation inside metals and is thus attributed to backscattered He neutrals interaction with the growing Al film. At very high ΓHe in the gas phase (above 95%), the role of these He fast neutrals is promoted since their number increases and the deposition rate decreases. Implantation leading to large bubble formation and emptying, as well as sputtering processes, can occur. At the same time, since the kinetic energy brought by Al depositing atoms rises, surface mobility is enhanced. These joint mechanisms lead to the formation of a fiberform or fuzz-like porous nanostructure. The presence of nanoparticles, not seen in He implantation experiments, could be explained by another Al deposition process: growth of nanoparticles in the gas phase that are deposited on the substrate. Complementary analyses of both the gas phase and the films are scheduled to deeper understand the growth processes in the three regimes evidenced in this first work. Regarding the application, films obtained in pure He gas will be tested for the hydrothermal synthesis of H2.

CRediT authorship contribution statement

  these trends suggest that different mechanisms of He trapping or desorption are involved depending on the plasma/deposition regime.The influence of He concentration on the structural properties of Al films deposited with different ΓHe was also investigated by GIXRD. As shown in Figure7a, all Al films exhibit the face-centered-cubic (fcc) crystal structure. The peaks corresponding to (111), (200), (220), (311), and (222) crystallographic planes are identified. The intensity of the peaks are similar to those given in the ICCD file (PDF card -00-004-0787) implying that Al films are polycrystalline and randomly oriented. Figure7bpresents a zoom on the angular 2 domain restricted around the peak corresponding to the (111) plane for selected films. The intensity of this peak decreases as ΓHe increases, indicating that the proportion of the diffracting material becomes lower. Two

  Sara: Conceptualization, Methodology, Writing, Review & editing. Fatima Zahrae: Formal analysis. Pascal: Methodology, Software and editing. Agnès: Formal analysis. Amaël: Investigation, Formal analysis and editing. Eric: Investigation, Formal analysis and editing. Thierry: Investigation, Formal analysis and editing. Asunción: Investigation, editing and Funding acquisition. Anne-Lise: Conceptualization, Supervision, Writing, Review & editing and Funding acquisition.

Figure 1 :

 1 Figure 1: Variation of the cathode voltage versus the percentage of He in the gas phase. Three

Figure 2 :

 2 Figure 2: Al deposition rate for different He percentages (ΓHe).

Figure 3 :

 3 Figure 3: Energy distribution functions of Al atoms at the substrate calculated by SRIM

Figure 4 :

 4 Figure 4: Experimental ion energy distribution function of (a) Al + , (b) Ar + and (c) He + ions

Figure 5 :

 5 Figure 5: (1,2,3 a) Cross-section and (1,2,3 b) top-view SEM images of Al films deposited with

Figure 6 :

 6 Figure 6: Evolution of the proportion of He to Al in the films as a function of the percentage of

Figure 7 :

 7 Figure 7: (a) GIXRD patterns of Al films and (b) zoom on the main peak (111) for films

Figure 8 :

 8 Figure 8: Evolution of the crystallite size of the different samples for an equivalent thickness

Figure 1 Figure 2 Figure 3 Figure 4

 1234 Figure 1

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Figure 8

 8 Figure 8

Acknowledgements

This work was supported by the region of « Centre Val de Loire » and the European Regional Development Funds (FEDER). The authors thank LE STUDIUM Loire Valley Institute for Advanced studies, and Asunción Fernández thanks also the support by the Spanish grant no. RTI2018-093871-BI00 (co-financed by EU FEDER).

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.


Gladys Barragan-Jason

Maxime Cauchoix

Anne Regnier

Marie Bourjade

Astrid Hopfensitz

Alexis S Chaine

Schoolchildren cooperate more successfully with non-kin than with siblings

Keywords: Kin selection, Kinship, Evolution of cooperation, Children development, Human Evolution

Cooperation plays a key role in the development of advanced societies and can be stabilized through shared genes (kinship) or reciprocation. In humans, cooperation among kin occurs more readily than cooperation among non-kin. In many organisms, cooperation can shift with age (e.g. helpers at the nest), however little is known about developmental shifts between kin and non-kin cooperation in humans. Using a cooperative game, we show that 3-to-10 yearold French schoolchildren cooperated less successfully with siblings than with non-kin children, whether or not non-kin partners were friends. Furthermore, children with larger social networks cooperated better and the perception of friendship among non-friends improved after cooperating. These results contrast with the well-established preference for kin-cooperation among adults and indicate that non-kin cooperation in humans might serve to forge and extend non-kin social relationships during middle childhood and create opportunities for future collaboration beyond kin. Our results suggest that the current view of cooperation in humans may only apply to adults and that future studies should focus on how and why cooperation with different classes of partners might change during development in humans across cultures as well as other long-lived organisms.

Introduction

Cooperation is thought to have played a key role in the evolution of advanced societies, especially in humans [START_REF] Melis | How is human cooperation different?[END_REF][START_REF] Dugatkin | Cooperation among animals: an evolutionary perspective[END_REF][START_REF] West | Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection[END_REF][START_REF] Clutton-Brock | Cooperation between non-kin in animal societies[END_REF][START_REF] Sachs | The evolution of cooperation[END_REF][START_REF] Fehr | Strong reciprocity, human cooperation, and the enforcement of social norms[END_REF][START_REF] Boyd | Culture and the evolution of human cooperation[END_REF]. Both kin-based interactions and reciprocity can promote cooperation while protecting against cheating [START_REF] Dugatkin | Cooperation among animals: an evolutionary perspective[END_REF][START_REF] West | Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection[END_REF][START_REF] Clutton-Brock | Cooperation between non-kin in animal societies[END_REF][START_REF] Sachs | The evolution of cooperation[END_REF][START_REF] Fehr | Strong reciprocity, human cooperation, and the enforcement of social norms[END_REF]. Cooperation among kin can mean that benefits to the recipient of help can lead to indirect genetic benefits to the donor, an evolutionary process called "kin-selection" [START_REF] Hamilton | The genetical theory of kin selection[END_REF], as described in social insects [START_REF] Queller | Kin selection and social insects[END_REF][START_REF] Hughes | Ancestral monogamy shows kin selection is key to the evolution of eusociality[END_REF] and a number of vertebrates [START_REF] Emlen | The role of kinship in helping decisions among white-fronted bee-eaters[END_REF][START_REF] Sinervo | Self-recognition, color signals, and cycles of greenbeard mutualism and altruism[END_REF]. Alternatively (or additionally), unrelated individuals who interact repeatedly can reciprocate leading to benefits to each individual in the partnership across time [START_REF] Trivers | The evolution of reciprocal altruism[END_REF][START_REF] Axelrod | The evolution of cooperation[END_REF] such as egg trading in fish [START_REF] Petersen | Reproductive behavior, egg trading, and correlates of male mating success in the simultaneous hermaphrodite, Serranus tabacarius[END_REF][START_REF] Fischer | The relationship between mating system and simultaneous hermaphroditism in the coral reef fish, Hypoplectrus nigricans (Serranidae)[END_REF] and allogrooming in primates [START_REF] Schino | The relative roles of kinship and reciprocity in explaining primate altruism[END_REF].

More recently, a few studies have shown that both mechanisms can operate in tandem with one mechanism playing a more important role in cooperation for a given system. For example, food sharing in vampire bats is clearly maintained by reciprocity even if such sharing can occur among kin adding further indirect benefits [START_REF] Carter | Social benefits of non-kin food sharing by female vampire bats[END_REF][START_REF] Carter | Social bet-hedging in vampire bats[END_REF][START_REF] Carter | Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment[END_REF]. Likewise, cooperation in humans occurs both among kin [START_REF] Vollan | The difference between kinship and friendship: (Field-) experimental evidence on trust and punishment[END_REF][START_REF] Krupp | A cue of kinship promotes cooperation for the public good[END_REF] as well as through reciprocal interactions [START_REF] Fehr | Strong reciprocity, human cooperation, and the enforcement of social norms[END_REF][START_REF] Milinski | Reputation helps solve the "tragedy of the commons[END_REF][START_REF] Forsythe | Fairness in Simple Bargaining Experiments[END_REF][START_REF] Wedekind | Cooperation through image scoring in humans[END_REF][START_REF] Reis | On mediating equity, equality, and self-interest: The role of self-presentation in social exchange[END_REF][START_REF] Haley | Nobody's watching?[END_REF], although human adults clearly favor cooperation with kin over strangers [START_REF] Smith | Inheritance of wealth as human kin investment[END_REF][START_REF] Stewart-Williams | Altruism among kin vs. nonkin: effects of cost of help and reciprocal exchange[END_REF][START_REF] Berté | K'ekchi'horticultural labor exchange: productive and reproductive implications[END_REF][START_REF] Crittenden | Juvenile foraging among the Hadza: Implications for human life history[END_REF].

In many organisms, the form of cooperation can change with age. For example, young individuals act as helpers in cooperative breeding birds but adults do not [START_REF] Emlen | Cooperative breeding in birds and mammals[END_REF][START_REF] Cockburn | Evolution of helping behavior in cooperatively breeding birds[END_REF] whereas the opposite is true in many cooperative breeding mammals [START_REF] Clutton-Brock | Review Lecture: Mammalian Mating Systems[END_REF]. Whether similar shifts occur with age in systems where both kin and non-kin cooperation occur either in absolute terms or in the relative importance of each form of cooperation, is unknown. Studying the development of kin and non-kin cooperation is critical to our understanding of the evolution and function of cooperation in longer-lived organisms and their role in the development and evolution of advanced societies [START_REF] Boyd | Culture and the evolution of human cooperation[END_REF][START_REF] Carballo | Cooperation and collective action: archaeological perspectives[END_REF]. Surprisingly, appropriate experimental tests in humans are still lacking. Among the limited number of experimental studies comparing kin and non-kin cooperation or prosociality in children, reciprocation seems to be important [START_REF] Sebastián-Enesco | Two and a half-year-old children are prosocial even when their partners are not[END_REF][START_REF] Warneken | The emergence of contingent reciprocity in young children[END_REF][START_REF] Hamlin | How infants and toddlers react to antisocial others[END_REF][START_REF] Martin | Beyond good and evil: what motivations underlie children's prosocial behavior?[END_REF] but children tend to favor sharing with their relatives compared to strangers [START_REF] Olson | Foundations of cooperation in young children[END_REF][START_REF] Lu | Resource allocation to kin, friends, and strangers by 3-to 6year-old children[END_REF] like adults. However, previous experimental studies of kin-sharing in children use third party tasks, where participants are asked about abstract scenarios, and these tests often give different results from direct participation in a task [START_REF] Bruce Morton | Are you listening? Exploring a developmental knowledge-action dissociation in a speech interpretation task[END_REF][START_REF] Tomasello | The role of ontogeny in the evolution of human cooperation[END_REF] since it reflects what the participant thinks another person should do but not necessarily how they would actually behave in a real cooperative situation (see however Crittenden et al. [START_REF] Crittenden | Juvenile foraging among the Hadza: Implications for human life history[END_REF]). In addition, third party tasks are thought to be "removed from the evolutionary mechanisms that [...] likely shape these phenomena in early ontogeny" and do not "reflect the effects of the collaborative foraging context of early humans, in which one shares the spoils […] among those who took part in the collaborative effort" [START_REF] Tomasello | The role of ontogeny in the evolution of human cooperation[END_REF]. Because no experimental study in children has directly compared kin and non-kin cooperation in situations in which individuals are asked to actively collaborate in a realistic setting, little is known about the development of kin and non-kin cooperation in humans with age.

Here, we used a direct-action cooperation task to evaluate if children cooperate more with kin, friends, or non-friends when actively engaged in a game that requires children to cooperate and coordinate actions with other children. We measured cooperation using a rope pulling task (Fig. 1A) in which two children coordinate in pulling a single rope to reach a reward and succeed only if both rope ends are pulled at the same time and same speed [START_REF] Plotnik | Elephants know when they need a helping trunk in a cooperative task[END_REF][START_REF] Rekers | Children, but not chimpanzees, prefer to collaborate[END_REF][START_REF] Warneken | Young children share the spoils after collaboration[END_REF]. This task is complex and needs the active engagement of children in the task since it requires paying attention to the partners' actions to succeed. While a previous experiment using this design began by giving the children a demonstration, we decided to render the task more difficult by omitting the demonstration and merely telling the children that they would have to work together to complete the task [START_REF] Warneken | Young children share the spoils after collaboration[END_REF]. To examine the roles of kinship and friendship on performance by children of a cooperative task, we assigned each child a partner who could be classified as kin (a sibling), a socially close non-kin (a "best friend") or a socially distant non-kin (a non-friend). Pairs were quasi-randomly assigned by the experimenter based on questionnaires administered to teachers and children before the task (see methods) and each pair was allowed to conduct the task until successful or up to three attempts if unsuccessful. In so doing, we investigated whether joint success in a cooperative task was linked to the degree of relatedness between partners in children. We found that children cooperate more efficiently with non-kin (friends and non-friends) than with kin (i.e. siblings) partners.

Methods

Participants

We recruited 290 children from ages 3 to 10 (92 3-to-5 year-olds, 139 6-7 year-olds and 59 8-to-10 year-olds; 135 females) from 15 kindergartens and elementary schools in southwestern France. While this age range is large, it reflects all children that potentially interact within schools where the study was conducted as many schools have mixed level classes and a common play area. All parents signed an informed consent form for their children and only children who gave their verbal assent were included. Parents also filled a demographic questionnaire including parents' income, living area (urban vs. rural), number of siblings and native language. Thirty percent of children were from middle-class backgrounds (20,000 to 30,000 euros/year) and 35 % lived in urban areas. Participants had 2.5 siblings on average: 18% were an only child, 44% had only one sibling, 20% had two siblings and 18% had more than 2 siblings. Sixty-nine percent of the children were native French and all children (except 2 children from whom the test was performed in English)

were French-speaking. The same female experimenter tested children during a single videorecorded session in an available room at their schools.

Experimental procedure

Participants performed a rope pulling task [START_REF] Plotnik | Elephants know when they need a helping trunk in a cooperative task[END_REF] that requires coordinated pulling to reach a reward (see Fig. 1 and Fig. S1). Two children are required to pull their end of the rope simultaneously, each holding the end of the same rope where the two ends are far enough apart that one person could not reach both ends at the same time. A single rope is threaded around an apparatus such that only if both rope ends are pulled at the same time can the containers be moved and the rewards be reached. Pulling on one end would only move the rope but not the two sliding containers which contain the rewards, making the other end of the rope unavailable to a partner. Only if both participants pulled the rope at the same time and at similar speeds, would they each obtain a reward (i.e. joint success, with no delayed reciprocity). Two cases lead to failure in the cooperative task: asymmetric pulling led to neither participant obtaining the reward (0/0) or if both participants pulled at the same time but one let go after gathering their reward but before their partner could take their own reward such that just one of the two obtained the reward (0/1 or 1/0). If only one child took his/her own sticker, the sticker was put back into the sliding container and children began a new trial together.

The Experimenter (E) explained to the children that they would play together to each win a reward (stickers) but provided no further instruction. By not providing more guidance, this cooperative task was rendered more difficult than in previous studies using the same game (63% success here vs. 94% success during first trials in a previous study [START_REF] Warneken | Young children share the spoils after collaboration[END_REF]). E placed the two rewards (one for each child) in the apparatus (one in each container of the apparatus) under observation of the children and told them they could start to play.

Each time a pair attempted to pull the ropes is termed a "trial". Our measure of cooperation and coordination was focused on a single trial, but upon request of teachers to not create inequalities in the classroom, dyads of children could perform a maximum of 3 trials beyond which E stopped the testing and gave the stickers to the children. As such, we focused our analyses on the first trials to avoid any impact of learning on joint success in subsequent trials. Overall, most children succeeded within those 3 trials as 63% (91/145) of the pairs succeeded in the first trial, 21% (31/145) in the second trial, 6% (9/145) during the third trial, and only 10 % (14/145) of dyads did not succeed by the 3 trial. In supplemental information, we provide additional analyses of these subsequent trials which reinforces results from the first trial only (see supplemental results, Table S1 and Fig. S2). Overall the sample included 39 pairs of siblings, 52 pairs of friends, and 54 pairs of non-friends who performed 77, 71 and 67 trials respectively, corresponding to a total of 215 trials among 145 dyads.

The partners of a dyad could be either siblings, friends (someone they frequently play and interact with), or non-friends (someone they know but do not particularly interact with).

The status of the dyad (i.e., siblings, friends, non-friends) was determined before conducting the experiment by asking the children's teachers to name the friends of the participants and specifying each participant's best friend through a questionnaire filled out before the experiment. We asked teachers to base their estimation of relationship closeness of a dyad on the amount of time children spent together, intensity of positive interactions, and time they play with each other at school [START_REF] Howes | Gender and friendship: relationships within peer groups of young children[END_REF][START_REF] Howes | The earliest friendships. The company they keep: Friendship in childhood and adolescence[END_REF]. Based on the responses of the questionnaires, dyads were formed by E. Following the definition of friendship in the literature [START_REF] Howes | Patterns of Friendship[END_REF], "friends" were formed by dyads where both children considered the other their best friend and "non-friends" were formed among two individuals where neither considered the other a friend. Furthermore, "non-friend" dyads specifically avoided matching individuals who have conflictual relationships. Finally, kin status was assigned to any siblings within the class or between two classes in the school. Because it was more rare to find siblings given logistical constraints at schools, possible sibling dyads were formed as a priority. Remaining individuals were then assigned to the other two categories at random. Given that kin dyads naturally showed larger age and sex differences than friend dyads (often same age and same sex), the "non-friend" dyads were balanced by matching both pairs of the same age and same sex children and pairs with large age differences and different sex to be more similar to the kin category (see Fig. S3).

In order to confirm the assignation of the dyads based on teacher evaluation of friendship, a questionnaire for children was also performed. E asked children about their relationship with their partner ("Do you like to play with CHILD X?") and about the quality of their relationship using an emoticon Likert scale ("How much do you like to play with CHILD X?: a lot, a little, not at all) [START_REF] Asher | A reliable sociometric measure for preschool children[END_REF]. The order of the emoticons was counterbalanced across participants to avoid bias. In order to investigate whether participation in the rope pulling task affected the relationship of the partners, the same questions were administered 24 h after the test. Finally, we gathered information about each child's friend network before the experiment, by asking the child to name their friends ("Please, tell me the names of the children you like to play with the most?"). Due to logistical constraints, we were only able to gather complete friendship network data at 10 of the 15 schools in our sample.

Data coding

All trials were recorded using a video camera oriented so that both participants and the apparatus were visible allowing us to score the children's performances. Successful trials were scored when both partners pulled together and successfully reached the reward (i.e. joint success). Failed trials were scored when children failed to pull the rope together such that neither reached the reward or when only one child reached the reward. The same trained research assistant coded the number of gazes (each movement of the eyes accompanied by a movement of the head toward the partner) of each dyad blind to dyad category for the first trial only of each dyad. The number of gazes included situations when both partners look at each other and when a single individual looks at the other one.

Statistical analysis

All analyses were performed in the R environment for statistical computing version 3.3.6 (R Development Core Team, 2018).

We examined the effect of partner relationship within a dyad on joint success using a binomial GLM [START_REF] Bates | Package "lme4[END_REF] including the first trial (0 vs. 1) performed by each dyad. We built a full model that included fixed effects of dyad relationship (kin, friend, non-friend), dyad sex (male-male (MM), female-female (FF), male-female (MF)), average age of partners and age difference between the partners. Since nearly all dyads eventually succeeded in performing the task by the third trial (see above), analyses of the change in relationship between a pair could not be compared between pairs that succeeded in the first trial relative to those who did not and so simply included all individuals.

Coordinated rope pulling could be facilitated by looking at a partner, so we looked at the impact of gaze frequency (gazes per second) on joint success in the first trial (0 vs. 1) performed by each dyad using a GLM. We built a full model that included fixed effects of dyad relationship (kin, friend, non-friend), dyad sex (MM, FF, MF), average age of partners and age difference between the partners. We also investigated the effect of partner status on the number of gazes using a LM including gaze frequency (gaze per second) during the first trial. We included dyad relationship (kin, friend, non-friend), dyad sex (MM, FF, MF), average age of partners and age difference between the partners as fixed effects.

We then assessed the relationship between performance in the task and the size of a child's social network. Using a binomial GLM, we asked whether performance (0 vs. 1) during the first trial was affected by a child's number of friends (i.e. outdegree centrality in social network analysis) while controlling for dyad sex, mean age of the dyad, age difference of the dyad partners, and number of children in the classroom.

For GLMs, visual inspection of residual plots using the DHARMa package [START_REF] Hartig | DHARMa: residual diagnostics for hierarchical (multi-level/mixed) regression models[END_REF] did not reveal deviations from homoscedasticity or normality. For each fixed effect, statistical significance was evaluated by likelihood ratio tests of the full model against the same model without the tested fixed effect. In supplemental tables, we report odd-ratios and their corresponding 95% confidence intervals, p-values as well as marginal, conditional or pseudo R2 of the Full Model when appropriate.

Finally, we tested the effect of the rope pulling task on the quality of the relationship between the two partners, we performed McNemar and Cochran Q test when appropriate on kin, friend and non-friend partners separately.

Results and discussion

During first trials, 63% (91 out of 145 trials) of the children jointly succeeded in the cooperative rope pulling task. We found that children cooperate less well with kin than with non-kin (Fig. 1B): the likelihood of cooperating during the first trial was significantly affected by dyad type while controlling for mean age, age difference and sex of dyads (binomial GLM, partner status: P=0.019, mean age: P=0.008; age difference: P=0.084, sex: P=0.81; Fig. 1B and Table S2). Friends (52 dyads, 71% successful; z=2.62; P=0.009) and non-friends (54 dyads, 68 % successful, z=2.29; P=0.022) were more likely to succeed on their first cooperative trial than kin partners (39 dyads, 44%, Fig. 1B and Table S2) whereas performance of friends and non-friends did not differ from each other (z=-0.61; P=0.54; Fig. S2). There was no effect of an interaction between the mean age of the dyad and partner status (mean age × partner status: deviance= -0.056; P=0.97) nor between age difference of the dyad and partner status (age difference × partner status: deviance: 1.22 P=0.54) suggesting that any shifts in cooperative preferences towards kin with age must occur beyond 8-10 years-old (Fig. S4). These patterns were consistent when controlling for demographic information (Table S3) and restriction to subsets of the data (Figs. S4,S5). [START_REF] Warneken | Young children share the spoils after collaboration[END_REF][START_REF] Hirata | Chimpanzees (Pan troglodytes) learn to act with other individuals in a cooperative task[END_REF]. Photograph of the apparatus is also provided in Fig. S1 in

1B and Table

Fig. 1. Performance in the cooperative rope pulling task by partner status. A) Illustration of the cooperation apparatus. The "rope pulling game" was adapted from previous studies on chimpanzees and children

supplemental information (SI). B) Proportion of successful first trials of kin (i.e. siblings) and non-kin friend and non-friend dyads in cooperative rope pulling. Error bars indicate 95% bootstrapped confidence interval. N values indicate the number of dyads in each category (i.e. kin, friends, non-friends). Kin partners were less successful than non-kin in the rope-pulling task.

The contrast in the propensity to cooperate with kin relative to non-kin between studies in adults [START_REF] Vollan | The difference between kinship and friendship: (Field-) experimental evidence on trust and punishment[END_REF][START_REF] Krupp | A cue of kinship promotes cooperation for the public good[END_REF][START_REF] Thomas | Kinship underlies costly cooperation in Mosuo villages[END_REF] and our results in children suggest that there is a striking developmental shift in the value of different forms of interactions. We could not run the same exact study in adults since the task would be far too easy for them and so could not directly contrast preferences in adults and children using the same population. However, studies in adults show a very consistent pattern of kin preferences over non-kin across a broad variety of experimental paradigms (e.g. public good game, prisoners' dilemma) and populations. For instance, adults from Western industrialized countries trust kin more [START_REF] Vollan | The difference between kinship and friendship: (Field-) experimental evidence on trust and punishment[END_REF], are more likely to cooperate with individuals who have similar facial features to their own [START_REF] Krupp | A cue of kinship promotes cooperation for the public good[END_REF], and minimize punishment after rule transgression for kin compared to strangers [START_REF] Lieberman | The effect of social category on third party punishment[END_REF][START_REF] O'gorman | Altruistic punishing and helping differ in sensitivity to relatedness, friendship, and future interactions[END_REF]. Furthermore, non-Western traditional societies show comparable preferences for cooperation with kin [START_REF] Chagnon | Evolutionary biology and human social behavior: An anthropological perspective[END_REF][START_REF] Smith | Inujjuamiunt foraging strategies: Evolutionary ecology of an Arctic hunting economy. Inujjuamiunt foraging strategies: Evolutionary ecology of an Arctic hunting economy 59[END_REF].

While we did find an improvement in joint success in the cooperative task with age (Fig. S4 and Tables S2 andS3), this improvement did not alter patterns of cooperation between kin versus non-kin (i.e. no significant interaction effect reported above; Fig. S4) while controlling for dyad age suggesting that contrasts between children and adults in their preference for cooperating with kin are not simply due to an improvement of solving a cooperative task.

Well known cooperative systems shift from cooperation among kin to no cooperation or the reverse with age [START_REF] Emlen | Cooperative breeding in birds and mammals[END_REF] [33] [START_REF] Clutton-Brock | Review Lecture: Mammalian Mating Systems[END_REF], but to our knowledge, such age related shifts in cooperative preferences have not yet been examined experimentally in systems where both kin and nonkin cooperation coexist. For example, it would be interesting to know if vampire bats which show reciprocal cooperation with non-kin more so than kin among adults show a strong preference for food sharing among kin rather than non-kin among juveniles. Knowing if such shifts are general will require both a concerted effort to document more cases of kin and nonkin cooperation in the same system as well as tests of cooperative preferences with age.

Shifts in preferences for cooperating with kin vs. non-kin may be a consequence of a shift in the value of cooperating with different classes of individuals with age. Kin cooperation among adults might provide the greatest direct and indirect benefits to success (e.g., fitness, wealth, etc. [START_REF] Chopik | Associations among relational values, support, health, and wellbeing across the adult lifespan[END_REF]) since they have reached reproductive maturity where gene transmission is likely more important than reciprocity thereby favoring kin interactions and indirect genetic benefits from cooperation. On the other hand, children are far from reproductive age and therefore might invest primarily in resource acquisition and survival which can benefit from reciprocal cooperation with peers regardless of kinship. Furthermore, kin-competition might reduce the value of kin-cooperation among children (e.g. siblings) [START_REF] Vehrencamp | A model for the evolution of despotic versus egalitarian societies[END_REF][START_REF] Rousset | Genetic structure and selection in subdivided populations (MPB-40)[END_REF][START_REF] Hamilton | Dispersal in stable habitats[END_REF] especially if resources are primarily provided by parents [START_REF] Tomasello | The role of ontogeny in the evolution of human cooperation[END_REF]. The benefits of a given cooperative interaction to success in children are indeed modest given that children are still supported by their parents and instead may serve primarily to develop cooperative skills needed for the future such as building a social network [START_REF] Kramer | Children's help and the pace of reproduction: cooperative breeding in humans[END_REF].

Developing friendships and affiliations with peers in mid-childhood has indeed been linked to future success at adulthood [START_REF] Chopik | Associations among relational values, support, health, and wellbeing across the adult lifespan[END_REF][START_REF] Gifford-Smith | Childhood peer relationships: social acceptance, friendships, and peer networks[END_REF][START_REF] Masten | The development of competence in favorable and unfavorable environments. Lessons from research on successful children[END_REF]. Since the current network of young children is still fairly limited, reinforcing and increasing reputation through reciprocity and building a broader social network might thus be more important during childhood than adulthood. Indeed, social networks tend to expand in size among young adults, but shrink in older adults [START_REF] Wrzus | Social network changes and life events across the life span: a meta-analysis[END_REF]. Here, we found that having a bigger social network before the experiment was related to subsequent performance during the first trial in the rope pulling task (Fig. 2) after controlling for the age difference between partners, mean age, sex and number of children in the classroom (binomial GLM, out degree centrality or number of friends named by participants: deviance=5.61; P=0.018, mean age: deviance=10.27; P=0.001; age difference: deviance=3.14; P=0.076, sex: deviance=2.12; P=0.35; number of children in the classroom: deviance=1.12; P=0.29; Fig. 3, Fig. 2, Table S4 and Fig. S6). This correlative relationship could exist either because social individuals cooperate more readily or because those who have built a bigger network develop cooperative skills. Regardless of directionality, our results show a cooperative benefit to a larger social network. Building a large social network should be especially valuable in unpredictable environments, since extending one's social network to cooperate with non-kin could provide benefits when the social community is perturbed whereas limiting one's social network only to kin would be risky [START_REF] Carter | Social benefits of non-kin food sharing by female vampire bats[END_REF][START_REF] Carter | Social bet-hedging in vampire bats[END_REF]. For example, under unpredictable/risky situations [START_REF] Carter | Social bet-hedging in vampire bats[END_REF][START_REF] Olson | A trait-based approach to understand the evolution of complex coalitions in male mammals[END_REF] or when non-kin are more numerous than kin [START_REF] Carter | Social benefits of non-kin food sharing by female vampire bats[END_REF], different species of mammals (e.g. bats [START_REF] Carter | Social bet-hedging in vampire bats[END_REF], primates and dolphins [START_REF] Olson | A trait-based approach to understand the evolution of complex coalitions in male mammals[END_REF]), tend to favor cooperation with non-kin compared to kin partners. For children, the school environment is constituted mostly of non-kin and has some risks (e.g. victimization by peers [START_REF] Boulton | BULLY/VICTIM PROBLEMS AMONG MIDDLE SCHOOL CHILDREN[END_REF][START_REF] Olweus | Bully/victim problems among schoolchildren: Basic facts and effects of a school based intervention program[END_REF][START_REF] Ladd | Classroom peer acceptance, friendship, and victimization: destinct relation systems that contribute uniquely to children's school adjustment[END_REF]), so expansion of the social network could indeed carry "social bet hedging" benefits. If cooperation serves to strengthen or build a social network, we would predict that participation in a cooperative action should alter future interactions. As such, we investigated whether performing the rope pulling task subsequently modified the relationship between the two partners. To do so, children were asked to rate their relationship with their partner before and again one day after the cooperative task (Fig. 3) using a Yes/No preference test ("Do you like to play with CHILD X?"; Fig. 3A) and an emoticon Likert scale ("How much do you like to play with CHILD X? A lot, a little, not at all"; Fig. 3B). We found that the relationship quality of non-friends improved after performing the rope pulling task together in both the Yes/No preference scale (McNemar χ2 (1,56) =9.60; P = 0.002; Fig. 3A) and the Emoticon Likert scale (Cochran Q(1,55)=7.35; P=0.007; Fig. 3B). As expected, we did not detect a change in how much children liked kin partners (Yes/No preference test:

McNemar χ2 (1,25 )=1.33; P=0.25; emoticon Likert scale: Cochran Q (1,25) =1.80; P= 0.18; Fig. 3) and friend partners (Yes/No preference test: McNemar χ2 (1,49) =1; P=1; emoticon Likert scale: Cochran Q (1,49) =2.78; P=0.10; Fig. 3) after the task since kin and friends already liked their dyad partners in nearly all cases before performing the cooperative task (Fig. 3). Responses from the Emoticon Likert scale during which children were asked to rate how much they like to play with their partner twice (before and after the rope pulling task). They can either respond "A lot" in light grey, "a little" in medium grey or "not at all" in dark grey while pointing to a "smiley scale" corresponding to each level. Perceived relationship quality of non-friends improved after cooperating.

Overall, our results show that cooperation between non-kin partners plays a key role during childhood which we argue serves to expand a child's social network since non-friends had a more positive view of their "unknown" partner after interacting during the cooperative task. These results contrast with two studies in which children had to choose how to allocate resources between dolls (i.e. third party tasks with fictional characters) which both showed greater apparent cooperation with kin than friends or strangers [e.g. [START_REF] Olson | Foundations of cooperation in young children[END_REF][START_REF] Engelmann | Young children care more about their reputation with ingroup members and potential reciprocators[END_REF]. We believe that differences in experimental methods lead to this contrast since the use of fictional characters is more likely to elicit a response that reflects what children think they or others should do [START_REF] Bruce Morton | Are you listening? Exploring a developmental knowledge-action dissociation in a speech interpretation task[END_REF][START_REF] Bélanger | What will I like best when i'm all grown up? preschoolers' understanding of future preferences[END_REF] whereas direct interactions to cooperate in a face-to-face situation used in our experiments should better reflect the actual outcome of natural cooperative situations. Furthermore, direct tasks are more challenging than simple allocation tasks such that costs of cooperation could also alter decisions. A difference in the value of friendships vs. sibling relationships among children relative to adults might drive contrasts in how much effort each group put into the cooperative task. The rope pulling task used here requires continued, active attention and coordination with a partner to succeed. Indeed, we observed that most of the unsuccessful trials happened either when a child pulled the rope before the other could grab it, or when a child let go of the rope before his/her partner could grab their own reward. We hypothesized that one possible explanation of failure could be a lack of attention between the two partners. Consistent with this idea, the number of gazes during the first trials was indeed a strong predictor of joint success (Fig. S7 and Tables S5) and kin dyads displayed fewer gazes than other types of dyads (friends partners: P < 0.005; non-friends: P=0.056; Fig. S7).

Using this direct-action cooperation task, we found that children cooperate more with non-kin peers compared to siblings in direct contrast with results in adults. Given the difficulty of our cooperative task for children and considering the time children in western societies spend at school, we believe our results reflect an ecologically realistic measure [START_REF] Tomasello | Origins of human cooperation and morality[END_REF] of cooperation in schoolchildren, at least in France. However, the value of cooperation with kin versus non-kin could vary by context or culture and it would be interesting to see if our results hold in other populations. Indeed, a more complete understanding of how decisions about cooperation shift through life will require both attention to the context of testing (e.g. [START_REF] Daly | Evolutionary social psychology and family homicide[END_REF]) and application of similar direct-interaction tests in individuals from a broad age range and across cultures.

Whether such developmental shifts in cooperation are common in other organisms also remains to be explored, but should exist in cases where the benefits of cooperating with different types of partners shifts through life [START_REF] Heinsohn | Reproductive skew[END_REF]. This new hypothesis motivated by our findings challenges our understanding of cooperation and should stimulate new research into cooperation across life stages in both humans and other social organisms.

Fig. 2 .

 2 Fig. 2. Relationship between the number of friends (out degree centrality) and performance in the rope pulling task. The number of friends and social network were based on a questionnaire before the experiment where children were asked to give the names of children they prefer to play with in their classroom including children who did not participate in the task. A) Boxplots contrasting the number of friends averaged between the two partners according to their performance during the first trial. Failure in the first trial is shown in red and joint success in green. Each dot represents a dyad of children. B) Examples of two classroom networks in which individuals who succeeded in the first trial appear in green and who failed in red. Children who did not participate and who participants named as friends appear in grey. Arrows represent friendship between children such that bi-directional arrows represent pairs of individuals who each listed the other as a friend whereas single headed arrows represent cases where one individual considered the other a friend while the second individual did not list the first as a friend. All networks are presented in Fig. S6. Having more friends is linked to a higher success in the cooperative task.

Fig. 3 .

 3 Fig. 3. Effect of the cooperative task on the relationship quality between kin, friend and nonfriend partners. A) Results from the Yes/No preference scale in which children were asked to answer the following question, before and after the rope pulling task: "Do you like to play with CHILD X?". "Yes" responses appear in light grey and "No" responses in dark grey. B)

We thank parents of participants, directors, teachers and inspectors of the schools for permission to work in schools and children who participated in the study. We thank Orlane Scelsi for her drawing of the apparatus, David Lieuré for help adapting and building the apparatus for children and Bruce Lyon and Michael Singer for helpful feedback on the manuscript. ANR-Labex TULIP (ANR-10-LABX-41) New Frontiers Grant entitled "Human Altruism Genes" and the Institute for Advanced Studies in Toulouse provided funding. This work was supported by the Laboratoire d'Excellence (LABEX) entitled TULIP (ANR-10-LABX-41) and IAST through ANR grant ANR-17-EURE-0010 (Investissements d'Avenir program).

Additional Information

Supplementary Information is available for this paper. Correspondence and requests for materials should be addressed to Gladys Barragan-Jason. Email: gladys.barraganjason@gmail.com