Apparatus and method for producing a uniform and regular periodic nanostructured pattern on the surface of a material

Description

Technical field

[0001] The present invention relates to the technical field of the formation of microstructures or nanostructures on the surface of solid materials.

[0002] More specifically, the invention relates to a system and method for nano-structuring or nano-texturing surfaces by laser.

[0003] Even more specifically, the invention relates to an apparatus and method for generating a periodic nanostructured pattern on the surface of a material. Prior art

[0004] Optical, electronic or ion lithography techniques allow the engraving of micrometric or nanometric patterns on the surface of a material in a predefined shape using a beam of photons, electrons or ions respectively. Lithography techniques can be applied to various metallic, semiconductor or dielectric materials. However, these manufacturing techniques require the use of expensive and complex machines. In addition, the application of lithography techniques on large surfaces (for example, on a 300 mm diameter silicon wafer) requires several manufacturing steps and very long manufacturing times.

[0005] In the above field, the technique of producing patterns by direct laser interference (or DLIP according to the English terminology) is known. The publication “Scanner-based Direct Laser Interference Patterning on Stainless Steel”, A. Madelung, S. Alamri, T. Steege, B. Krupop, AF Lasagni, and T. Kunze, Adv. Eng. Mater., 2021, 2001414, describes a DLIP apparatus comprising a laser source generating a beam of laser pulses, a diffractive optical element (DOE) for spatially separating the source beam into two partial beams, a prism for parallelizing them and an optical system for focusing and superimposing the two partial beams so that they interfere on the surface to be structured. In DLIP, laser pulses generally have a duration that can vary between a few hundred femtoseconds (fs) and a few tens of nanoseconds (ns).The DLIP system allows the generation of periodic structures on the surface of a metallic material, for example stainless steel. When two partial beams interfere at an angle of incidence ALPHA, a periodic intensity distribution is produced with a spatial period P1 defined according to the following equation: . P 1 = λ 2 sin ALPHA , where λ represents the wavelength of the laser pulses and ALPHA the angle of incidence of each of the two laser beams on the surface. For example, documents DE 102015214960 (describing the preamble of claims 1 and 8), CN 109926712 or US 2014 / 291308 each describe an apparatus for structuring the surface of a flat sample by laser interference with at least two beams.

[0006] With two partial beams, the DLIP technique can thus generate a pattern of periodic lines. The wavelength of the laser beam being for example 1030 nm and the angle of incidence ALPHA being between 10.2 and 2.3 degrees, a DLIP periodic structure with a period P1 between 2.9 µm and 12.8 µm is thus obtained. The periodic pattern is generated on the interference zone of the partial beams, i.e. generally on a diameter of less than a hundred micrometers. To extend the structured surface, the publication "Scanner-based Direct Laser Interference Patterning", A. Madelung et al. proposes combining the DLIP interferometric system with a scanner placed upstream of the optical focusing system. The two-mirror galvanometric scanner can move the beam interference zone on the surface to be treated in two dimensions. The scanning or translation speed must be sufficient to maintain the desired coverage.However, the scanner aperture limits the minimum spatial period P1 to 2.9 µm. To obtain a pattern with a period less than 500 nm, in DLIP, a laser emitting in the green or UV range must be used.

[0007] On the other hand, the technique for producing laser-induced periodic surface structures (or LIPSS) is also known. The LIPSS technique is based on the use of a single continuous or pulsed laser beam with a sufficiently high fluence. In the case of a pulsed laser, the duration of the laser pulses is generally in the femtosecond or picosecond range (for example 1 ps, 10 ps, ​​30 ps or 40 ps). A lens focuses the pulse beam onto the surface to be treated so that the pulse beam has a fluence greater than the ablation threshold of the material. The applied fluence is, for example, of the order of 0.1 J / cm 2< to structure a stainless steel surface using LIPSS. The LIPSS technique makes it possible to induce periodic patterns whose orientation depends on the polarization of the laser beam.More specifically, since the laser beam is linearly polarized, the pattern features are aligned perpendicular to the polarization of the laser beam. Periodic patterns have a spatial period greater than half the laser wavelength (in the case of high spatial frequency LIPSS, or HSFL) and less than or equal to the laser wavelength (in the case of low spatial frequency LIPSS, or LSFL). The extent of the LIPSS-induced patterns is limited by the size of the laser beam on the surface to be treated. Patent document US 2014 / 0083984 discloses a LIPSS apparatus comprising a two-dimensional galvanometric scanner for scanning the laser beam over the surface to be treated. However, the LIPSS-induced patterns are generally not spatially homogeneously organized at the sub-millimeter scale.Furthermore, LIPSS remains difficult to control due to its high sensitivity to material type and laser irradiation parameters, which impairs the reproducibility and uniformity of periodic patterns. The physical phenomena underlying laser-induced surface patterns are not yet well understood. However, LIPSS surface patterns appear to originate from an interaction between the incident laser beam and surface electromagnetic waves, or surface plasmons, excited by the incident radiation.

[0008] One of the aims of the invention is to propose a system and a method for surface nano-structuring or nano-texturing, which makes it possible to generate periodic patterns of sub-micrometric or nanometric period, these patterns being regular over a large surface, i.e. over a surface with dimensions of the order of several cm 2< or several tens of cm 2<.

[0009] Another aim of the invention is to propose a system and a method for surface nano-structuring or nano-texturing, which is rapid, simpler and less expensive than the solutions of the prior art, in particular lithography. Statement of the invention

[0010] The invention relates more specifically to an apparatus for generating a periodic nanostructured pattern on a surface of a material as defined in claim 1, the apparatus comprising a laser source adapted to generate a source pulse beam at a wavelength λ, an optical beam splitter arranged to spatially separate the source pulse beam into a first secondary beam and a second secondary beam, a first optical system configured to direct the first secondary beam and the second secondary beam parallel to the same direction with a non-zero separation distance, a focusing optical system having an aperture adapted to receive the first secondary beam and the second secondary beam,the optical focusing system being capable of focusing and superimposing the first secondary beam and the second secondary beam on the same area of ​​the surface of the material at an angle of incidence ALPHA in an incidence plane, the first secondary beam and the second secondary beam being linearly polarized with a polarization state in the incidence plane. According to the invention, the source pulses have a duration of between 100 fs and 100 ps, ​​and the first secondary beam and the second secondary beam superimposed spatially and temporally in said area have a fluence greater than the ablation threshold of the material, the angle of incidence ALPHA and / or the fluence being adjusted so as to induce a periodic nanostructured pattern extending in a direction transverse to the incidence plane, the periodic nanostructured pattern having in the incidence plane a spatial modulation period equal to , P 2 = P 1 N where N is an integer greater than or equal to 2 and P1 represents a spatial period inherent to the interference between the first secondary beam and the second secondary beam defined by P 1 = λ 2 . sin ALPHA .

[0011] The apparatus and method make it possible to generate a periodic, regular, homogeneous nanostructured pattern with a spatial period equal to an integer fraction of the spatial period P1 obtained by conventional DLIP interference. The apparatus and method make it possible to generate this nanostructured pattern with a submicrometric or nanometric period organized homogeneously over a large area and in a limited time.

[0012] Using a picosecond laser makes the device and process less expensive than a system based on a femtosecond laser.

[0013] In an example application, the target fluence range is less than or equal to 2.1 J / cm 2< .

[0014] According to a particular and advantageous aspect, the apparatus comprises means for adjusting the polarization of the source pulse beam and / or means for adjusting the polarization of the first secondary beam and of the second secondary beam.

[0015] Advantageously, the first optical system comprises a variable optical delay line for synchronizing the first secondary beam and the second secondary beam.

[0016] In an exemplary embodiment, the optical beam splitter comprises a splitter blade.

[0017] Advantageously, the first optical system comprises a translationally adjustable prism for adjusting the separation distance between the first secondary beam and the second secondary beam.

[0018] According to the invention, the apparatus comprises a scanner arranged between the first optical system and the focusing optical system, the scanner being configured to scan the surface of the material with the first secondary beam and the second secondary beam focused on said surface, along one or two directions transverse to the normal to the surface of the material at the focusing point.

[0019] Alternatively or additionally, the apparatus comprises a displacement plate adapted to move the material in at least one direction transverse to the normal to the surface of the material at the focal point.

[0020] Advantageously, the laser source comprises a laser emitting at a wavelength between 200 nm and 1100 nm, for example between 343 nm and 1064 nm.

[0021] According to the invention, the optical focusing system comprises an F-Theta lens of focal length f, having an aperture greater than or equal to 0.8*f and / or whose focal length f is less than 40 mm.

[0022] The invention also relates to a method for generating a periodic nanostructured pattern on the surface of a material as defined in claim 8, the method comprising the steps of: generating a beam of source pulses at a wavelength λ, the source pulses having a duration of between one hundred femtoseconds and one hundred picoseconds; spatial optical separation of the beam of source pulses into a first secondary beam and a second secondary beam; orientation of the first secondary beam and the second secondary beam parallel to the same direction with a non-zero separation distance d;focusing and superimposing the first secondary beam and the second secondary beam on the same area of ​​the surface of the material with an angle of incidence ALPHA in an incidence plane, the first secondary beam and the second secondary beam being linearly polarized with a polarization state in the incidence plane, the first secondary beam and the second secondary beam, spatially and temporally superimposed in said area, having a fluence greater than the ablation threshold of the material, the angle of incidence ALPHA and / or the fluence being adjusted so as to induce a periodic nanostructured pattern extending in a direction transverse to the incidence plane, the periodic nanostructured pattern having in the incidence plane a spatial modulation period equal to: ; P 2 = P 1 N ,where N is an integer greater than or equal to 2 and P1 represents a spatial period inherent to the interference between the first secondary beam and the second secondary beam defined by P 1 = λ 2 . sin ALPHA .

[0023] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Brief description of the drawings

[0024] In addition, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting embodiments of the invention and where: There figure 1 is a schematic view of an apparatus according to the invention, The figure 2is an image obtained by scanning electron microscope (SEM) of a nano-structured surface according to the present disclosure, on an area of ​​approximately 50µm*50µm, with the Fourier transform of the SEM image inserted; The figure 3 is an SEM image of the same nano-structured surface as the figure 2 , with higher magnification; The figure 4 represents the angular distribution of the nanostructured patterns extracted from the figure 3 ; There Figure 5 is a profile view obtained by AFM of the nanostructured patterns, the profile being taken along a line transverse to the direction of the lines of the pattern of the figure 3 .

[0025] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0026] There figure 1represents a device 100 for generating nanostructured patterns. An orthonormal XYZ reference frame is also represented.

[0027] The apparatus 100 mainly comprises a pulsed laser source 1, an optical beam splitter 2, a prism 28, a scanner 30 and an optical focusing system 31. Advantageously, the apparatus 100 comprises a device 13, 14, 15 for adjusting the polarization of the laser beams.

[0028] The laser source 1 is for example a laser emitting pulses at a wavelength λ, generally in the range of 343 to 1064 nm. The pulses emitted by the laser form a beam of source pulses 10. In our example, the laser 1 is a Sirius laser (Spark brand). The wavelength λ is 1064 nm, with a spectral width of approximately 0.3 nm. The duration of the source pulses is greater than 100 fs and less than 100 picoseconds (ps), for example 10 ps. The pulse repetition frequency is between 1 Hz (single pulse mode) and 1 MHz, for example here 30 kHz.

[0029] The laser used in this example has a power of approximately ~< 2.4 W, a maximum target fluence of approximately ~< 2.1 J / cm 2< and linear polarization.

[0030] Optionally, one or more plane mirrors M1, M2 allow the source beam to be folded and directed towards an optical beam splitter 2.

[0031] The optical beam splitter 2 spatially separates the source pulse beam 10 into a first secondary beam 11 and a second secondary beam 12. The optical beam splitter 2 here consists of a splitter plate, for example a fused silica plate, with flat and parallel faces, 5 mm thick. Optionally, a quarter-wave plate 13 (or λ / 4 plate) is arranged upstream of the splitter plate 2 to equally distribute the energy of the source pulse beam 10 between the first secondary beam 11 and the second secondary beam 12.

[0032] A set of plane mirrors 21, 22, 23, 24, 25 directs the first secondary beam 11 towards one face of a right prism 28. Another set of plane mirrors 26, 27 directs the second secondary beam 12 towards another face of the prism 28. After reflection on the prism 28, the first secondary beam 11 and the second secondary beam 12 are parallel to the same direction, for example the direction X on the figure 1 , while being spaced by a non-zero distance d. The plane mirrors 21, 22, 23, 24, 25, 26, 27 and the prism 28 here form a first optical system for orienting the first secondary beam and the second secondary beam parallel to the same direction. Alternatively, two other plane mirrors can be used instead of the prism 28.

[0033] Advantageously, a device 29 for adjusting the separation distance d between the first secondary beam 11 and the second secondary beam 12 comprises the prism 28 mounted to move in translation along the direction X. Indeed, the translation of the prism 28 along the direction X makes it possible to adjust the distance d between the first secondary beam 11 and the second secondary beam 12.

[0034] In addition, the mirrors 22 and 23 are mounted integrally movable in translation, for example in the Z direction so as to form a variable optical delay line 20. The adjustment in position of the mirrors 22 and 23 makes it possible to adapt the optical path difference between the first secondary beam 11 and the second secondary beam 12 and to synchronize the first secondary beam 11 with the second secondary beam 12 on the surface 32 of the material 33 to be treated.

[0035] A half-wave plate 14 (or λ / 2 plate) is arranged on the optical path of the first secondary beam 11 to adjust the polarization orientation of the first secondary beam 11. Similarly, another half-wave plate 15 is arranged on the optical path of the second secondary beam 12 to adjust the polarization orientation of the second secondary beam 12.

[0036] Optionally, a plane mirror M3, oriented here at 45 degrees relative to the X direction, simultaneously changes the directions of the first secondary beam 11 and the second secondary beam 12 to orient them in the Z direction towards an optical focusing system 31. The first secondary beam 11 and the second secondary beam 12 remain parallel to each other and separated by the distance d up to the entrance of the scanner 30 and the optical focusing system 31.

[0037] The optical focusing system 31 comprises an F-theta type lens, with a focal length f. An F-theta lens is chosen having a short focal length and a very large aperture entrance pupil which makes it possible to receive beams spaced apart by a transverse distance d of the order of 10 mm to 24 mm, for example 24 mm. For example, the F-theta type lens has a focal length f and an entrance pupil whose diameter varies between k1f and k2f with k1 = 0.80 and k2 = 0.93.

[0038] The optical focusing system 31 simultaneously focuses and superimposes the first secondary beam 11 and the second secondary beam 12 on the same area 32 of the surface of the material 33 to be treated. It is considered here that the figure 1represents the plane of incidence, i.e. the plane containing the first secondary beam 11 and the second secondary beam 12 and perpendicular to the area 32 of the surface of the material 33 to be treated. As described above, the first secondary beam 11 and the second secondary beam 12 are incident on the F-theta lens, while being parallel to the optical axis of the F-theta lens and each being eccentric by a distance d / 2 with respect to this optical axis. The surface 32 of the material to be treated is arranged in the focal plane of the F-Theta lens, the optical axis of the F-theta lens being perpendicular to the surface 32 to be treated. In other words, the optical axis of the F-theta lens, the first secondary beam 11 and the second secondary beam 12 are located in the plane of incidence. The focusing lens thus makes it possible to spatially superimpose the first secondary beam 11 and the second secondary beam 12 on an area 32 of the material 33 to be treated.

[0039] The half-wave plate 14 (or λ / 2 plate) and / or the other half-wave plate 15 are adjusted in orientation so that the first secondary beam 11 and the second secondary beam 12 are both linearly polarized in the plane of incidence.

[0040] Finally, adjusting the distance d between the first secondary beam 11 and the second secondary beam 12 makes it possible to adjust the angle of incidence of these beams on the zone 32. The first secondary beam 11 forms an angle of incidence +ALPHA on said zone 32 and, respectively, the second secondary beam 12 forms a symmetrical angle of incidence -ALPHA on said zone 32.

[0041] Optionally and particularly advantageously, the apparatus comprises a scanner 30 arranged between the mirror M3 and the optical focusing system 31. The scanner 30 is configured to scan the surface of the material with the first secondary beam 11 and the second secondary beam 12 focused on said surface, in one or two directions transverse to the normal to the surface of the material at the focusing point. By way of non-limiting example, the scanner comprises two plane mirrors mounted on galvanometric actuators and arranged in series on the path of the first secondary beam 11 and the second secondary beam 12. One of the galvanometric mirrors makes it possible to scan the surface to be treated in the X direction and the other galvanometric mirror in the Y direction. Advantageously, the scanner 30 has an entrance pupil of 30 mm.

[0042] Alternatively or additionally, the apparatus comprises a displacement stage 34 adapted to move the material 33 in at least one direction transverse to the normal to the surface of the material 33 at the focal point 32. For example, the displacement stage 34 is a two-dimensional translation stage in XY or a three-dimensional translation stage in XYZ. The stage makes it possible to move the surface of the sample relative to the incident beams to treat areas outside the field of the galvanometric scanner and / or to treat non-adjacent areas by a combined scanning and stitching technique. Process

[0043] A beam of source pulses 10 is generated. The source pulses 10 have a pulse duration greater than 100 fs and less than 100 picoseconds (ps), for example 10 ps. The maximum spectral width of the source pulses 10 is limited to approximately 5 nm, so as not to risk deteriorating the quality of the interference in terms of contrast and spatial homogeneity. The source pulses 10 are emitted at a repetition frequency of 30 kHz. The source pulses 10 have a wavelength λ of 1064 nm. The source pulse beam 10 is spatially separated into a first secondary beam 11 and a second secondary beam 12 by means of a splitter plate 2. Advantageously, the energy distribution between the first secondary beam 11 and the second secondary beam 12 is adjusted so that they have approximately the same energy, for example by orienting the quarter-wave plate 13 (or λ / 4 plate) arranged upstream of the splitter plate 2.Optical components with mirrors 21, 22, ...27 and a straight prism 28 are used to direct the first secondary beam 11 and the second secondary beam 12 parallel to the same direction, for example the X direction, while spacing them by a non-zero distance d, for example 24 mm. Advantageously, the prism 28 is moved to adjust the distance d. In addition, a variable optical delay line 20 is used to synchronize the pulses of the first secondary beam 11 and the second secondary beam 12. For example, the optical delay line displacement plate makes it possible to adjust the synchronization to within ~< 70 fs, or ~< 0.7% of the pulse duration in this case.

[0044] The first secondary beam 11 and the second secondary beam 12 are focused and spatially superimposed on the same area 32 of the surface of the material 33 to be treated. The focal distance is for example 30 mm. The polarization of the first secondary beam 11 and / or the second secondary beam 12 is adjusted so that they have the same polarization when they are incident on the area to be treated. In this way, the first secondary beam 11 and the second secondary beam 12 are spatially and temporally superimposed with the same polarization on the area to be treated. The first secondary beam 11 and the second secondary beam 12 can thus interfere in the area 32 of the surface of the material 33 to be treated.

[0045] More precisely, the first secondary beam 11 and the second secondary beam 12 are linearly polarized in the plane of incidence. As described in more detail below, this configuration makes it possible to induce LIPSS-type nanostructures arranged between the nanostructures generated by DLIP, the LIPSS nanostructures having the same orientation as the nanostructures generated by DLIP, in other words to orient the lines of the LIPSS-type nanostructures parallel to the DLIP-type interferences. The fluence received by the zone 32 is greater than the ablation threshold of the material. In the example illustrated in FIGS. 2 to 6, the fluence is between 70 mJ / cm 2 < and 100 mJ / cm 2 < . As an indication, the ablation threshold of stainless steel is approximately: ~ < 0.1 J / cm 2 < .

[0046] In accordance with the invention, a scanner 30 is used to scan the surface of the material 33 with the interfering beams over a field of ±5 degrees. In the examples illustrated in Figures 2 to 6, the average power of the focused laser beam is of the order of a hundred milliwatts.

[0047] The present disclosure makes it possible to obtain a nanostructuring or nanotexturing of the area 32 of the surface of the material 33 with a periodic pattern at a LIPSS period, P2, which is associated with or determined by the DLIP period P1. In the prior art, the nanostructures induced by LIPSS (of the low spatial frequency LIPSS type) always have a period close to the laser wavelength λ. According to the present disclosure, the DLIP type nanostructures formed by interference between the secondary beams 11 and 12 make it possible to overcome this limit by forming LIPSS type nanostructures having a period P2 less than half the wavelength of the pulses, in other words P2 <λ / 2. In addition, the DLIP configuration makes it possible to orient the nanostructures in a precise and regular manner. More precisely, a pattern of period P2 equal to an integer fraction of the period P1 of the DLIP interference is thus obtained.To the inventors' knowledge, such a phenomenon has apparently not yet been observed and was not foreseeable.

[0048] The periodic patterns obtained are very regular. According to the invention, the scanner extends the nanostructured area periodically by scanning the surface of the sample, while superimposing the focusing spots, with a spatial overlap of at least 50%, for example 60% or 80%. This produces a periodic pattern with a period P2, which is less than half the wavelength of the laser beam.

[0049] Without being bound by a physical interpretation of the phenomena involved, it seems that there is an interaction between DLIP interference at period P1 and surface plasmons to induce additional interference or harmonics between two features of the interference network at period P1. Depending on the applied fluence, a periodic pattern can be obtained at a period P2 equal to P1 / 2 or P1 / 3. In certain configurations, notably by increasing the period P1, it is possible to obtain a periodic pattern at a period P2 equal to P1 / 4.

[0050] Figures 2 to 6 illustrate an example of application of the apparatus and method of the present disclosure. The material to be treated here is a flat sample of stainless steel. However, the apparatus and method are in no way limited to this example of application. The apparatus and method of the present disclosure make it possible to form nanostructures on other metallic or metallized materials or even on non-metallic materials, such as semiconductors or dielectric materials.

[0051] The wavelength of the source pulse beam here is 1064 nm and the angle of incidence ALPHA of the secondary beams 11, 12 is 22.3 degrees.

[0052] By classical DLIP interference, periodic structures are obtained having a period P1 which is determined by the angle of incidence ALPHA and the wavelength of the incident laser beams, according to the formula indicated above.

[0053] In one example, a wavelength λ of 1064 nm, a beam spacing d of 24 mm and a focal length of 30 mm lead to a period P1 of DLIP interference of approximately 1.4 µm.

[0054] According to the present disclosure, more restricted operating conditions are used than the conventional DLIP technique, which makes it possible to obtain nanostructures with a smaller spatial period than conventional DLIP nanostructures. In particular, the fluence and the number of pulses in the interference zone are adjusted here.

[0055] In the example illustrated in Figures 2 to 6, the fluence applied to a mirror-polished stainless steel sample is between 70 mJ / cm 2 and 100 mJ / cm 2 . Under these conditions, between 6 and 10 passes are applied, which corresponds to a treatment time of approximately ~ 4 s / mm 2 . The passes are carried out on the same area, in the example described here at a repetition frequency of 30 kHz, with an overlap in the scanning direction (horizontal) of approximately 88% and a vertical overlap of approximately 75%. This produces on the surface of the material a pattern of periodic lines oriented perpendicular to the plane of incidence and therefore to the direction of polarization of the secondary beams 11, 12. The structured pattern has a single spatial period P which is very uniform over the treated surface.Unexpectedly, the period P2 of the pattern obtained is much lower than the period P1 obtained conventionally in DLIP and also much lower than the expected LIPSS period (of the order of the wavelength, here, 1064 nm). More precisely, we observe that the periodic pattern obtained by the invention has a spatial period P2 equal to an integer fraction of the period P1. In other words, the obtained spatial period P2 is equal to . P 2 = P 1 N = λ 2 N sin ALPHA where N is an integer greater than or equal to 2. In the example illustrated in Figures 2 to 6, P2 is equal to 470 nm. In other words, P2 is here equal to P1 / 3, in this case the integer N is equal to 3.

[0056] It is possible to obtain a periodic pattern with N=2, by adjusting the fluence between 130 mJ / cm 2< and 150 mJ / cm 2< after a single pass and with the same spatial overlap as for N=3, which corresponds to a cumulative energy dose between 3.4 J / cm 2< and 4 J / cm 2< .

[0057] To obtain a pattern with a period P2 with N greater than 3, it is preferable to increase the period P1, by changing the angle of incidence of the secondary beams 11, 12. Changing the wavelength here simultaneously changes the LIPSS and DLIP periods. To change only the DLIP period, it is then necessary to change the angle of incidence ALPHA of the interfering beams.

[0058] Changing the wavelength simultaneously and proportionally changes the LIPSS period, P2, and the DLIP period, P1.

[0059] Unlike the conventional low spatial frequency LIPSS structure formation technique, which uses only a single laser beam, the period of the resulting pattern is not close to the wavelength of the laser beam but less than half the wavelength. More precisely, in the example described above, P2 is equal to 44% of the wavelength.

[0060] The pulse duration is between 100 fs and about 100 ps, ​​and for example between 1 ps and 10 ps. On the one hand, too large an emission bandwidth (>5 nm) does not allow to obtain interferences with a spatially uniform contrast, for the machining of homogeneous structures. On the other hand, the pulse duration less than 100 ps makes it possible to avoid thermal effects likely to melt the induced periodic patterns.

[0061] The secondary beams 11, 12 are focused on an area 32 having a diameter of approximately 60 µm. To extend the surface of the treated area, the scanner 30 is used to scan the surface with the secondary beams 11, 12 in an interference configuration.

[0062] The scanner here has a very large aperture, of the order of 30 mm, to allow the secondary beams 11, 12 separated by the distance d to pass through, while allowing scanning over a large field, for example ± 5 degrees. The scanner scan is carried out successively in the X and Y directions. In the scanning direction, the textured surface was obtained with 8.4 pps / pass, where pps means pulse per spot with a spatial overlap of 88%, and a number of passes varying between 6 and 10. In the direction perpendicular to the scan, the overlap amounts to 75%. These conditions are equivalent to a cumulative energy dose of between 11 and 26 J / cm 2 < .

[0063] There figure 2is a scanning electron microscopy image of a surface treated according to the method and with the apparatus of the present disclosure. The pattern of periodic lines oriented perpendicular to the plane of incidence and to the polarization of the incident secondary beams 11, 12 is observed.

[0064] Inserted on the figure 2 , a Fourier transform of the SEM image is shown, which indicates that the nanostructured metal surface has a well-defined period, P2, of 470 nm with an angular distribution δθ of about 5.1 degrees. The period P1 at 1.4 µm is also observed, which is less intense than the period at 470 nm. This angular distribution of about 5 degrees is much narrower than that obtained by the previous LIPSS technique (greater than 20 degrees). This narrow angular distribution shows that the periodic pattern has a unique, well-defined orientation.

[0065] Scanning the secondary beams 11, 12 superimposed with an overlap between spots greater than 50% makes it possible to obtain a homogeneous texturing over the entire treated surface, for example 0.5 mm x 0.5 mm. The scanning speed of the scanner is 0.2 m / s. The scanner makes it possible to nanostructure the surface with a speed of a few seconds per mm 2< at a repetition frequency of 30 kHz.

[0066] There figure 3 shows an enlargement of the figure 2 on an area of ​​the sample. The arrow represents the direction of polarization of the secondary beams 11, 12, in the XZ plane of incidence.

[0067] There Figure 5 represents a profile of the features acquired by AFM following a line transverse to the features. The Figure 5 shows that the nanostructures are highly modulated, with a modulation depth between 100 nm and 250 nm. The modulation period P2 is very uniform over the treated surface.

[0068] This process allows a surface to be treated simply, using a single laser and a galvanometric scanner, and to obtain a periodic pattern with a period of less than 500 nm. This produces an iridescent and colored metallic surface. The apparent color varies in particular depending on the period of the pattern and the orientation of the sample. It is easy to adjust the period by varying the distance between the beams, which modifies the angle of incidence on the surface to be treated.

[0069] The present disclosure finds applications in many industrial sectors such as electronics, household appliances, aeronautics, naval, medical, or even anti-counterfeiting. The possibility of generating periodic or regular structures at the nanometric or sub-micrometric scale on the surface of different types of materials, makes it possible in particular to functionalize this surface. It is thus possible to modify the optical properties of the surface, for example to obtain an anti-reflective treatment or to give an apparent color to the treated material. The treatment can also be used to improve the catalytic, antibacterial, hydrophobic and / or tribological properties of the surface. The treated surface can have new properties, for example antifouling, antibacterial and / or anti-virus.

[0070] The disclosed apparatus and method enable the generation of a homogeneous, highly regular surface nanostructuring. The pattern orientation can be easily defined based on the orientation of the plane of incidence and the polarization of the interfering secondary beams.

Claims

1. Apparatus (100) for generating a periodic nanostructured pattern on a surface of a material (33), wherein the apparatus (100) comprises a laser source (1) designed for generating a beam of source pulses (10) with a wavelength λ, an optical beam splitter (2) designed for spatially splitting the beam of source pulses (10) into a first secondary beam (11) and a second secondary beam (12), a first optical system (21, 22, 23, 24, 25, 26, 27, 28) designed for orienting the first secondary beam (11) and the second secondary beam (12) parallel to a same direction with a splitting distance different from zero, an optical focusing system (31) with an opening designed for receiving the first secondary beam (11) and the second secondary beam (12), the optical focusing system (31) being able to focus and overlay the first secondary beam (11) and the second secondary beam (12) to a same area (32) of the surface of the material (33) with an angle of incidence ALPHA in a plane of incidence, the first secondary beam (11) and the second secondary beam (12) being linearly polarized with a polarization state in the plane of incidence, characterized in that the apparatus comprises a scanner (30) located between the first optical system (21, 22, 23, 24, 25, 26, 27, 28) and the optical focusing system (31), the optical focusing system (31) comprising a F-Theta lens having a focal length f with an opening greater than or equal to 0,8•f and the focal length f being smaller than 40 mm, the splitting distance ranging from 10 mm to 24 mm, the scanner being designed for scanning the surface of the material with the first secondary beam (11) and the second secondary beam (12) which are focused on that surface over a field of ± 5 degrees, in one or two directions transverse to the direction perpendicular to the surface of the material at the focusing point and meanwhile overlapping the focusing spots with a spatial overlapping of at least 50 %, the source pulses have a duration ranging from 100 fs to 100 ps, and in that the spatially and timely overlapped first secondary beam (11) and second secondary beam (12) in that area have a fluence that is greater than the ablation threshold of the material, the angle of incidence ALPHA and / or the fluence being adjusted so as to induce a periodic nanostructured pattern extending in a direction transverse to the plane of incidence, the periodic nanostructured pattern having a spatial modulation period P2 in the plane of incidence that is equal to an entire fraction of a spatial period P 1 : P 2 = P 1 N , where N is an integer greater than or equal to 2 and P1 represents a spatial period included in interferences between the first secondary beam and the second secondary beam defined by P 1 = λ 2 sin ALPHA ..

2. Apparatus (100) according to claim 1 comprising means (13) for adjusting the polarization of the beam of source pulses (10) and / or means (14, 15) for adjusting the polarization of the first secondary beam (11) and the second secondary beam (12).

3. Apparatus (100) according to one of claims 1 to 2, wherein the first optical system (21, 22, 23, 24, 25, 26, 27, 28) comprises a variable optical delay line (22, 23) for synchronizing the first secondary beam (11) and the second secondary beam (12).

4. Apparatus (100) according to one of claims 1 to 3, wherein the optical beam splitter (2) comprises a splitting plate.

5. Apparatus (100) according to one of claims 1 to 4, wherein the first optical system (21, 22, 23, 24, 25, 26,27, 28) comprises a prism (28) translationally adjustable for adjusting the splitting distance between the first secondary beam (11) and the second secondary beam (12).

6. Apparatus (100) according to one of claims 1 to 5 comprising a moving plate (34) designed for moving the material (33) in at least one direction that is transverse to the direction perpendicular to the surface of the material (32) at the focusing point (32).

7. Apparatus (100) according to one of claims 1 to 6, wherein the laser source (1) comprises a laser emitting at a wavelength between 200 nm and 1100 nm.

8. Method for generating a periodic nanostructured pattern on a surface of a material (33), the method comprising the steps of: generating a beam of source pulses (10) at a wavelength λ, the source pulses having a duration ranging from 100 femtoseconds to 100 picoseconds; optical spatial splitting of the beam of source pulses (10) into a first secondary beam (11) and a second secondary beam (12); the method being characterized by the following steps: orienting the first secondary beam (10) and the second secondary beam (12) parallel to a same direction with a splitting distance different from zero and ranging from 10 mm to 24 mm; by means of a F-Theta lens having a focal length f with an opening greater than or equal to 0,8•f and the focal length f of which is smaller than 40 mm, focusing and overlaying the first secondary beam (11) and the second secondary beam (12) to a same area (32) of the surface of the material (33) with an angle of incidence ALPHA in a plane of incidence, the first secondary beam (11) and the second secondary beam (12) being linearly polarized with a polarization state in the plane of incidence, the spatially and timely overlapped first secondary beam (11) and second secondary beam (12) in that area having a fluence that is greater than the ablation threshold of the material, scanning the surface of the material with the first secondary beam (11) and the second secondary beam (12) which are focused on that surface over a field of ± 5 degrees, in one or two directions transverse to the direction perpendicular to the surface of the material at the focusing point and meanwhile overlapping the focusing spots with a spatial overlapping of at least 50 %, the angle of incidence ALPHA and / or the fluence being adjusted so as to induce a periodic nanostructured pattern extending in a direction transverse to the plane of incidence, the periodic nanostructured pattern having a spatial modulation period P2 in the plane of incidence equal to an entire fraction of a spatial period P 1 : P 2 = P 1 N , where N is an integer greater than or equal to 2 and P1 represents a spatial period included in interferences between the first secondary beam and the second secondary beam defined by P 1 = λ 2 sin ALPHA .