Anti-reflective, super-hydrophobic optical element with good mechanical strength

Abstract

The invention relates to an optical element (10) transparent at at least one operating wavelength (λ0), antireflective and superhydrophobic, comprising a sub-wavelength array of primary microstructures (MS1) made of a first material (M1) and formed on a substrate (Sub), the primary microstructures having a conical or frustoconical shape, a first width (L1) and a first height (H1) such that a ratio between the first width and the first height is less than 1 / 2, and an array pitch (p1) smaller than the first height (H1) of said primary microstructure, the array of primary microstructures thus having a structured surface (S), said structured surface being at least partially covered by a plurality of secondary microstructures (MS2) having a shape of nano-pillars or nano-cones and being made of a second material different from the first material,The second material exhibits a hardness strictly greater than that of the first material. Figure 3,

Description

Title of the invention: Anti-reflective, superhydrophobic optical element with good mechanical strength FIELD OF THE INVENTION

[0001] The present invention lies in the field of optical windows, and more particularly anti-reflective coatings dedicated to optical components or windows. More particularly, the invention relates to a transmission optical element having anti-reflective (AR), super-hydrophobic (SH) properties and exhibiting good mechanical resistance. STATE OF THE ART

[0002] Conventionally, anti-reflective coatings are obtained in different ways: thin layer, stack of thin layers, or by surface structuring at a sub-wavelength scale.

[0003] AR coatings are optimized for a given wavelength of use X0 or even a given spectral band (in which X0 is included). Subwavelength (called sub-X) is understood to mean structures having, as a first approximation, a size smaller than the XO / ns ratio, ns being the index of the substrate of the coating or of the window. Subsequently, these sub-X structures are called microstructures or plots, the English term being SWS for "sub-wavelength structures".

[0004] A very relevant approach is the use of a network of conical or truncated conical microstructures MS as illustrated [Fig.l] which synthesize an index gradient perpendicular to the substrate so as to achieve a progressive adaptation of the index. The sub-X microstructures form an artificial material having an effective index neff. By network of microstructures we mean the pattern or the spatial arrangement of the pads on the surface of a Sub substrate.

[0005] We define p as the pitch of the array of pads. The arrangement of the pads can be random or periodic. In the first case, we define the pitch p as the minimum distance between the center of any pad in the array and the center of its nearest neighbor. By "periodic" array of microstructures, we mean pads that repeat with a fixed pitch of ±10%.

[0006] The parameters of the plot are its width L and its height H. It is made of a Mat material which can be identical to the material of the substrate or different from it, depending on the materials and the manufacturing technology used.

[0007] From a practical point of view, however, it is not easy to obtain a perfect gradient varying from 1 (air index) to the substrate index ns. A surface filling rate of 100% is in fact not possible with cones, even with a mesh hexagonal, which allows at best a theoretical filling rate of 0.9 when the cones are joined at their base. In practice, the maximum rate achievable is around 0.7. The use of pyramids allows a surface filling rate of 1 but the realization of such structures is difficult and depends on the material, the realization of cones is preferred.

[0008] It is also desirable to produce components having a hydrophobic or superhydrophobic surface. Such a property can be achieved by structuring the surface quite finely. The roughness or structuring of the surface has the effect of trapping air in the structure, and a drop of water then rests on a composite surface made of solid and air. This effect, commonly called the "fakir" effect, makes it possible to achieve high contact angles (-160°) and a fairly low contact angle hysteresis (less than 10°).

[0009] It is known to use periodic SWS to confer superhydrophobic (SH) and anti-reflective (AR) properties to a glass transparent in the visible (Park KC et al. “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity” ACS Nano. 6(5):3789-99; 2012).

[0010] Optical surfaces using SWS are therefore ideal candidates for producing camera ports, binoculars or anti-reflective windows in observation systems in maritime environments for example. However, these microstructures are rapidly degraded in difficult operating conditions (rain, hail, sand erosion, etc.) which greatly limits the duration of the SH and AR properties of these optical elements in these environments.

[0011] In order to solve this problem, document WO2020 / 178304 proposes to conformally deposit a thin, homogeneous CP layer of a material having a significant hardness, greater than the hardness of the Mat material, as illustrated [Fig.2]. The role of this CP layer is to make the SWS resistant to harsh environmental conditions. The hard material is chosen in this document from: alumina, preferably in sapphire phase, DLC (for "Diamond Like Carbon") or ZrO2. An example of an embodiment is an AR structure on the 8-12 pm band with conical microstructures of height 3.2 pm in germanium and pitch 1.6 pm, covered with a layer of annealed alumina. The conformal layer protects the microstructures, and it is considered that the SH and AR properties are maintained.

[0012] However, it turns out that the deposition of a layer of a “hard” material on the network of cones tends in certain cases to disturb the optical properties of the surface and thus to reduce its anti-reflection capacity.

[0013] It should also be noted that since the hard layer has a constant index and the hard materials compatible with a given cone network are very limited, the addition of this layer does not make it possible to improve the optical and hydrophobic performances of the cone network, at most at best it retains the performance of the initial component without the layer.

[0014] An aim of the present invention is to overcome the aforementioned drawbacks by proposing an optical element made of anti-reflective, super-hydrophobic SWS, still having high mechanical strength and improved performance. DESCRIPTION OF THE INVENTION

[0015] The present invention relates to an optical element transparent to at least one wavelength of use, anti-reflective and super-hydrophobic, comprising an array of sub-wavelength primary microstructures made of a first material and formed on a substrate, the primary microstructures having a conical or truncated conical shape, a first width and a first height such that a ratio between the first width and the first height is less than U2, and a pitch of the array less than the first height of said primary microstructure, the array of primary microstructures thus having a structured surface,

[0016] said structured surface being at least partially covered with a plurality of secondary microstructures having a shape of nano-pillars or nano-cones and being made of a second material different from the first material, the second material having a hardness strictly greater than the hardness of the first material.

[0017] According to one embodiment, the secondary microstructures cover substantially the entire structured surface.

[0018] According to one embodiment, the network of primary microstructures is periodic, according to a square or hexagonal mesh.

[0019] According to one embodiment, a surface density of the secondary microstructures in the covered areas is determined so that an effective index of the plurality of secondary microstructures varies monotonically in a vertical direction perpendicular to the plane of the substrate.

[0020] According to one embodiment, a surface density of the secondary microstructures in the covered areas is determined so that an effective index of the plurality of secondary microstructures is between the index of air and the index of the substrate.

[0021] According to one embodiment, the second material is chosen from: diamond, alumina, DLC, SiN.

[0022] According to one embodiment, the second material is boron-doped diamond.

[0023] According to one embodiment, the optical element further comprises a layer of second material arranged on the primary structures and on which the secondary microstructures are arranged.

[0024] According to one embodiment, the hardness of the second material is greater than or equal to 1.3 times the hardness of the first material, the hardness being measured on the Knoop scale.

[0025] According to one embodiment, the wavelength of use is included in the visible or near infrared, and the hardness of the second material is greater than or equal to 3 times the hardness of the first material, the hardness being measured on the Knoop scale.

[0026] According to one embodiment, the wavelength of use is included in the MWIR (3-5 pm) or LWIR (8-12 pm) band and the hardness of the second material is greater than or equal to 5 times the hardness of the first material, the hardness being measured on the Knoop scale.

[0027] According to one embodiment, the secondary microstructures are arranged in a forest or in grass.

[0028] The invention also relates to a method for manufacturing an optical element transparent to at least one wavelength of use, anti-reflective and superhydrophobic.

[0029] According to a first variant, the method comprises the steps consisting of: • having a network of sub-wavelength primary microstructures made of a first material and formed on a substrate, the primary microstructures having a conical or truncated conical shape, a first width and a first height such that a ratio between the first width and the first height is less than / 2, and a pitch of the network less than the first height of said primary microstructure, the network of primary microstructures thus having a structured surface, • deposit on the structured surface a layer of a second material different from the first material and having a hardness strictly greater than that of the first material, • etching said structured surface by plasma, ionic, chemical etching or a combination of these techniques so that the structured surface is at least partially covered with a plurality of secondary microstructures having a shape of nano-pillars or nanocones.

[0030] According to one embodiment, the etching is a reactive ion etching.

[0031] According to a second variant, the method comprises the steps consisting of: • have a network of sub-wavelength primary microstructures made of a first material and formed on a substrate, the primary microstructures having a conical or truncated conical shape, a first width and a first height such that a ratio between the first width and the first height is less than / 2, and a pitch of the network less than the first height of said primary micro structure, the network of primary micro structures thus having a structured surface, • depositing by evaporation or cathodic sputtering a second material at an oblique incidence relative to the normal to the substrate so that the structured surface is at least partially covered with a plurality of secondary microstructures (MS2) having the shape of nanopillars or nano-cones, the network of primary microstructures being in rotation during the deposition step, the second material being different from the first material and having a hardness strictly greater than that of the first material.

[0032] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0033] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which:

[0034] The [Fig.l] already cited illustrates anti-reflective and super hydrophobic microstructures known from the state of the art.

[0035] The [Fig.2] already cited illustrates anti-reflective, super hydrophobic microstructures with good mechanical strength known from the state of the art.

[0036] [Fig.3] illustrates an optical element according to the invention with conical primary microstructures.

[0037] [Fig.4] illustrates an optical element according to the invention with truncated primary microstructures.

[0038] [Fig.4bis] illustrates an embodiment of the optical element according to the invention in which the optical element comprises a layer of second material arranged on the primary structures and on which the secondary microstructures are arranged.

[0039] [Fig.5] illustrates the simulated optical transmission as a function of the wavelength of an element composed of silica alone and for 4 angles of incidence 0°, 45°, 60° and 70°.

[0040] [Fig.6] illustrates the simulated optical transmission as a function of the wavelength of a structure with cones alone according to the state of the art for these same 4 angles of incidence 0°, 45°, 60° and 70° and for unpolarized light.

[0041] [Fig.7] illustrates the simulated optical transmission as a function of wavelength of a structure with cones coated with a homogeneous layer of diamond of 150 nm of thickness according to the state of the art according to the state of the art for these same 4 angles of incidence 0°, 45°, 60° and 70° and for unpolarized light.

[0042] [Fig.8] illustrates the division of the structure into layers indexed i with a thickness of 25 nm carried out for the simulation. Layer No. 1 is the air layer, then follow 6 layers (No. 2 to 7) of diamond for the case [cones + homogeneous diamond] or 6 layers of air for the case [cones alone], then 20 layers (No. 8 to 27) for the cones themselves in silica (and the diamond layer if applicable), and a layer (No. 28) for the substrate Sub also in silica.

[0043] [Fig.9] illustrates examples of layers for the structure comprising only the cones (horizontal section plane according to the layer).

[0044] [Fig. 10A] illustrates examples of layers for the structure comprising the cones covered with a homogeneous layer of diamond (horizontal section plane according to the layer), for layers no. 1, no. 2, no. 8 and no. 13.

[0045] [Fig.10B] illustrates examples of layers for the structure comprising the cones covered with a homogeneous layer of diamond (horizontal section plane according to the layer), for layers no. 14, no. 21, no. 27 and no. 28.

[0046] [Fig. 11] illustrates the variation of the effective index for both cases of cones alone and cones with diamond layer, as a function of the layer number.

[0047] [Fig. 12] illustrates the transmission of the optical element according to the invention comprising a forest of pillars with an equivalent effective index of 1.3.

[0048] [Fig. 13] illustrates the optical transmission of the structure comprising cones alone, for cones arranged in a pitch of 200 nm.

[0049] [Fig. 14] illustrates the optical transmission of the structure comprising cones covered with a homogeneous layer of diamond, for cones arranged in a pitch of 200 nm.

[0050] [Fig. 15] illustrates the optical transmission of the structure comprising cones covered with a forest of diamond pillars with an effective index of 1.8, for cones arranged in a pitch of 200 nm.

[0051] [Fig. 16] illustrates the optical transmission of the structure comprising cones covered with a forest of diamond pillars with an effective index of 1.3, for cones arranged in a pitch of 200 nm.

[0052] [Fig. 17] illustrates a method of producing the optical element according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0053] By “vertical” is meant a Z direction perpendicular to the surface of the substrate.

[0054] By "aspect ratio" R of an object we mean the ratio between its width L and its height H. By width, we mean the largest dimension in the plane tangent to the surface and by height H we mean the largest dimension perpendicular to the tangent to the surface: R=L / H

[0055] By “transparent” is meant here a transmission greater than 50%, preferably 75% and even more preferably 95% at a wavelength or over a range of illumination wavelengths.

[0056] By "nano-pillars" we broadly mean a nanostructure elongated in a direction D and which may have a cylindrical shape, solid or hollow, but also a zigzag, a helix, etc. The direction of elongation D is either normal to the surface on which the nano-pillar is formed or a direction inclined relative to the plane of this surface.

[0057] The optical element 10 according to the invention is illustrated in Figures 3 and 4. The optical element 10 is transparent at at least one wavelength of use X0, or in a spectral band of use BS0 in which X0 is included. The element 10 is also anti-reflective for the wavelength of use (or the band BS0) and super hydrophobic.

[0058] The element 10 comprises a network of primary microstructures MSI made of a first material Ml and formed on a substrate Sub, the primary microstructures having a conical ([Fig.3]) or truncated ([Fig.4]) shape.

[0059] The shape of the primary microstructures or pads is defined by a first width L1 and a first height H1 and they are separated by a first pitch pi.

[0060] The microstructures are sub-X and we therefore have as a first approximation:

[0061] Ll <X0 / ns, ns indice du substrat Sub.

[0062] Strictly speaking, for a square mesh we have aji < —2—, where 0 is the angle of incidence of the illumination.

[0063] In the case of a hexagonal mesh, we have Ll<~ s 77 -— •

[0064] More generally, a condition allowing different types of mesh and illumination is:

[0065] £l<1.5i

[0066] The conical or truncated shape, combined with the sub-X character of the primary microstructures, ensures the anti-reflective function (see state of the art).

[0067] The arrangement of the plots is random or periodic. In the first case, the pitch pl is defined as the minimum distance between the center of any plot in the network and the center of its nearest neighbor. In the second case, by "periodic" network of microstructures we mean plots that are repeated with a fixed pitch of ±10%. The periodic arrangement is typically according to a square or hexagonal mesh (best filling rate).

[0068] The aspect ratio RI between the width L1 and the height H1 is less than / 2 and the pitch of the network pl is less than the first height H1:

[0069] RI = L1 / H1 < 1 / 2

[0070] pl < H1

[0071] These two properties are necessary to obtain the hydrophobic character of the element. Indeed the pitch pl and the aspect ratio RI directly impact the contact angle between the water and the structured surface (see state of the art).

[0072] The network of primary microstructures thus presents a structured surface S, consisting of the surface of the cones and the inter-cone surface. The set of cones constitutes the primary structure of the element 10. Overall, this primary structure is similar to that described previously in the state of the art in [Fig.l].

[0073] For good AR and SH behavior, a high density of plots and a low aspect ratio (< 1 / 2, preferably <1 / 3, preferably < 1 / 6) are sought. For a non-periodic network, the minimum distance from a cone to its nearest neighbor is preferably between H1 / 3 and 2.H1 / 3.

[0074] According to one embodiment, the cones of the primary microstructures are contiguous.

[0075] The first surface density DSI, or surface filling rate, of the first microstructures MSI is defined as the proportion of the total surface area of ​​the substrate occupied by the base of the cones MSI. In the case of a hexagonal mesh of cones and contiguous cones, it is theoretically 0.9, but is practically limited to 0.7.

[0076] In the optical element 10 according to the invention, the structured surface S is furthermore at least partially covered with a plurality of secondary microstructures MS2 having a shape of nano-pillars or nano-cones. These pillars or these cones are for example nanowires (NW in English) or nanospikes (Nanospikes in English) made of a material M2 different from the material ML. Subsequently, the generic term “nano-pillar” will be used, which groups together the different possible shapes of the secondary micro / nano structures.

[0077] According to one embodiment, the arrangement of the microstructures MS2 is quite compact and random, and is then commonly called “forest” or “grass”.

[0078] The secondary microstructures constitute the so-called secondary structure arranged on the primary structure. The element 10 thus has a so-called hierarchical structure.

[0079] These nano-pillars typically have a width L2 of between 1 and 50 nm and a height H2 of between 50 nm and 500 nm, or more (forests of nano-pillars with a height of the order of a micron are achievable). They preferably have an aspect ratio R2 < 1 / 5, or even 1 / 10 and up to 1 / 100 or more. In the case of a “forest”, the arrangement of the nano-pillars is not periodic but random and the nano-pillars are not necessarily all identical and / or of the same height.

[0080] Finally, the second material M2 has a hardness strictly greater than the hardness of the first material M1 of the pads. The hardness of the materials M1 and M2 is understood here for the solid material (“bulk”).

[0081] Quite counter-intuitively, the inventors have established that replacing a homogeneous hard layer covering the element (see state of the art) with a structured assembly of a smaller scale than the MSI structures, also called secondary microstructures, makes it possible to maintain good hardness of the element. The presence of the forest of nano-pillars ensures good mechanical protection of the primary structure.

[0082] It also turns out that this secondary structuring on a scale smaller than the primary structuring makes it possible to obtain better optical results than the homogeneous hard layer, and therefore to overcome its drawbacks. In fact, the secondary structuring behaves like a secondary artificial layer having an effective index intermediate between air and the material Ml and / or the substrate.

[0083] Furthermore, this second level of structuring increases fluidic performance, in particular increases hydrophobicity.

[0084] Thus the optical element according to the invention produces a coating i) having reinforced mechanical resistance in order to limit erosion / abrasion problems when implementing the optics in a harsh environment ii) having an increased AR property in angle of incidence compared to the homogeneous hard layer iii) having reinforced hydrophobicity.

[0085] According to one embodiment, the first material Ml is identical to the material of the substrate. This is for example the case when the cones are directly produced on a substrate by masking and etching. According to another embodiment, Ml is different from the material of the substrate.

[0086] The invention applies in spectral bands of use such as visible / near infrared, the MWIR band (3-5 pm) or LWIR (8-12 pm). The dimensions of the MS 1 and MS2 structures as well as the pitch of the MS 1 network are adapted according to the spectral band of use.

[0087] For the visible / near IR the substrate and the MSI structures are for example made of silica or BK7, and the material M2 is typically chosen from alumina (preferably annealed into sapphire) or diamond.

[0088] For the MWIR band [3-5 pm] the substrate / MSl is for example silicon and the material M2 is typically chosen from alumina (preferably annealed into sapphire), diamond, DLC (“Diamond Like Carbon”), SiN.

[0089] For the LWIR band the substrate / MS 1 is for example germanium and the material M2 is typically chosen from diamond or DLC.

[0090] Examples of hardness of different materials are given in Table I below. Material Hardness of K noop (kg / m m2). Fused silica 500 N-BK7 610 Silicon 1150 Germanium 780 Diamond 7000 Sapphire 2200 DLC 1026100 SiN 1500-2000 Table I

[0091] For reinforced protection, the second material preferably has a hardness greater than or equal to 1.3 times the hardness of material Ml, the hardness being measured on the Knoop scale (kg / mm2).

[0092] With diamond on glass (silica or N-BK7) we see that the hardness ratio is greater than or equal to 10. For sapphire on glass this ratio is greater than or equal to 3. Thus in the visible / near IR band the hardness ratio is preferably greater than or equal to 3.

[0093] With diamond on germanium we see that the hardness ratio is greater than or equal to 8. For DLC (with an accessible hardness of at least 4000) on germanium this ratio is greater than 5. Thus in the LWIR band the hardness ratio is preferably greater than or equal to 5.

[0094] For the MWIR band, silicon being quite hard, the hardness ratio is preferably greater than or equal to 1.3 for the case of SiN and 5 in the case of diamond.

[0095] Thus, a particularly well-suited M2 material for the production of MS2 is diamond, which has a very high hardness (very good mechanical resistance) and with which a forest of nano-pillars can be produced using methods compatible with large surfaces and curved surfaces. Diamond is also transparent across all spectral bands of interest.

[0096] According to one embodiment, the diamond is doped with Boron, which ensures a self-cleaning character to the element.

[0097] According to a preferred embodiment, the secondary microstructures MS2 cover the entire structured surface, i.e. the surface of the cones and the intercone surface. This is the best configuration for hardness (the cones are protected), hydrophobicity, or superhydrophobicity.

[0098] According to an illustrated embodiment [Fig.4bis], for the non-limiting case in which the substrate and the primary microstructures are in the same material M1, the optical element comprises a layer LM2 in second material M2 arranged on the primary structures, and on which the secondary microstructures are arranged. The layer LM2 is for example a residual layer resulting from the manufacturing process of the secondary microstructures and which covers the entire surface S. Its thickness is typically between a few nanometers and a few hundred nm.

[0099] The properties of the optical element according to the invention are highlighted in the example below.

[0100] We consider an AR structure in the visible / near IR on the band [400 nm - at least Ipm], the cones and the substrate (Ml) are made of silica (index ns = ni = 1.45), the cones are arranged according to a periodic hexagonal mesh of period pl = 150 nm. The cones are contiguous and we therefore have L1 = pl = 150 nm with DSI = 0.9. The height H1 = 500 nm, we therefore have RI = 0.3. The forest is made of diamond nanowires (n2 = 2.4). The effective index of the forest neff / f is linked to the surface density of filling DS2 of the nanowires on the covered surface.

[0101] The transmission as a function of the wavelength (spectral band [0.4-1 pm]) of an element composed of silica alone is recalled for memory in [Fig.5] for 4 angles of incidence 0°, 45°, 60° and 70° for unpolarized incident light. The incident light is unpolarized. At 70° the transmission falls below 85%.

[0102] [Fig.6] illustrates the simulated transmission of the structure with the MS cones alone for these same 4 angles of incidence. The transmission remains above 98% except for 70° where it is at 92%. The AR character appears clearly in comparison with [Fig.5].

[0103] [Fig.7] illustrates the transmission with the MS cones covered with a layer homogeneous CP diamond layer (nd = 2.4) 150 nm thick according to the state of the art. In comparison with [Fig.6] the transmission drops, very noticeably at 70°. Thus the presence of the homogeneous diamond layer disrupts the AR function and degrades its performance.

[0104] The simulation was carried out by cutting the structure into layers indexed i of 25 nm and calculating for each layer the corresponding effective index. As illustrated [Fig.8] layer n°1 is the air layer, then follow 6 layers (n°2 to 7) of diamond for the case [cones + homogeneous diamond] or 6 layers of air for the case [cones alone], then 20 layers (no. 8 to 27) for the cones themselves in silica (and the diamond layer if applicable), and one layer (no. 28) for the Sub substrate also in silica.

[0105] Examples of layers for the case [cones alone] are illustrated [Fig.9]. Examples of layers for the case [cones + homogeneous diamond] are illustrated [Fig.10A] (layers no.1, no.2, no.8 and no.13) and [Fig.10B] (layers no.14, no.21, no.27 and no.28). These are horizontal sections according to the layer in question.

[0106] [Fig. 11] illustrates the variation of the effective index for the two cases as a function of the layer number i. The effective index varies monotonically for the case of the cone alone (curve 10) and has a break point for the case [cones + homogeneous diamond] (curve 20) corresponding to the zone noted 80 on [Fig.8].

[0107] [Fig. 11] also illustrates the evolution of the effective index when the homogeneous diamond layer is replaced by a forest of pillars with an effective index related to the surface filling density of the pillars, so as to form an element 10 according to the invention. Three effective indices neff / f of 1.3 (curve 13), 1.5 (curve 15) and 1.8 (curve 18) are studied, corresponding respectively to a surface filling density of 35%, 52% and 72%.

[0108] Curve 13 is the one that makes it possible to obtain a monotonic variation of the effective index neff / f along a vertical axis (i.e. perpendicular to the substrate plane), which leads to the best optical performances. Thus, preferably, the density DS2 is determined so that the effective index neff / f has a monotonic variation along the vertical direction.

[0109] According to an embodiment making it possible to ensure a monotonic variation, the surface density DS2 of the secondary microstructures in the covered zones is determined so that its effective index is between the index of air and the index of the substrate ns: 1 < neff / f < ns

[0110] [Fig. 12] illustrates the transmission of the element 10 according to the invention comprising a forest of pillars with an equivalent effective index neff / f of 1.3. The performances are very clearly improved compared to [Fig.7] (with homogeneous layer of diamond) but also slightly improved compared to [Fig.6] (cones alone).

[0111] Thus the presence of a forest of diamond nano-pillars makes it possible both to reinforce the mechanical strength of the component and to fully restore the optical performance of the anti-reflection, or even to improve it compared to cones without coating.

[0112] Figures 13, 14, 15 and 16 illustrate another advantage of the nanostructure forest.

[0113] They show the optical transmission of the structure respectively [cones alone] ([Fig.13]), [cones + homogeneous diamond] ([Fig.14]), [cones + forest / neff / f = 1.8] ([Fig.15]), [cones + forest / neff / f = 1.3] ([Fig.16]) but with cones arranged at a pl pitch of 200 nm (instead of 150 nm for Figures 5-7). In terms of manufacturing, more widely spaced cones are easier to manufacture. Performance is maintained with the forest, and an effective index of 1.8 also produces good results. We also note in [Fig. 15] that for this cone pitch additional interference effects further degrade the optical transmission of the case [cones + homogeneous diamond]. Thus with the secondary micro / nanostructures we have relaxed certain constraints on the pitch of the arrangement of the cones in relation to the homogeneous layer.

[0114] Methods for producing “grass” or “forest” nanopillars have been described for flat surfaces, and are applicable to the structured surface S of the primary structure. Methods for producing primary and secondary microstructures according to the invention can also be carried out on curved substrates.

[0115] The grass or forest of nano-pillars or nano-spikes can take different shapes and structures depending on the production method used.

[0116] According to another aspect, the invention relates to a method of manufacturing an optical element transparent at a wavelength of use, anti-reflective and super-hydrophobic.

[0117] For this, in a first step, it is appropriate to have a network of primary microstructures MSI in a first material Ml and formed on a substrate Sub. The primary microstructures have a conical or truncated shape, a width L1 and a height H1 such that Ll / Hl < V2, a pitch of the network pl

[0118] For the case in which the cones are made in the substrate material, this step amounts to micro / nano-structuring the network of pads on the surface of the substrate. ​

[0119] According to a first variant, a layer of a second material M2 different from the first material M1 and having a hardness strictly greater than that of the first material is then deposited on the structured surface. Different deposition techniques can be used, for example a CVD (Chemical Vapor Deposition) or ALD (Atomic Layer Deposition) type deposition. Preferably, the deposited layer is homogeneous and covers the entire structured surface of the cones (surface of the cones and inter-cone surface).

[0120] Then the structured surface covered with the M2 layer is etched by plasma, ionic, chemical etching or a combination of these techniques, so that the structured surface is at least partially covered with a plurality of secondary microstructures MS2 having a shape of nano-pillars or nanocones.

[0121] Preferably, once the secondary microstructures have been produced, a “silanization” step is carried out to further increase the hydrophobicity. This step typically consists of depositing a layer of silane on the optical element. Increased hydrophobicity can also be achieved by depositing a layer of a fluorinated compound.

[0122] According to one embodiment, the etching is a reactive ion etching.

[0123] The paper by Yoon et al “Insights into the reactive ion etching mechanism of nanocrystalline diamond films as a function of film microstructure and the presence of fluorine gas”, Journal of Applied Physiscs 107, 044313 (2010), describes a method for producing a diamond grass on silicon. The diamond layer is deposited by CVD, then etched by ICP (Inductively Coupled Plasma) with a dioxygen plasma. In this paper the process was implemented on a flat silicon substrate but this method is applicable to a silicon substrate structured by cones, as illustrated [Fig. 17]: A: deposition of the diamond layer CD on the structured surface S; B: etching to obtain the nano-pillars. In C is illustrated the diamond nano-pillar grass obtained by the aforementioned publication.

[0124] According to a second variant, the so-called GLAD technology for Glancing Angle Deposition is used. In this variant, the second material M2 is deposited by evaporation or cathodic sputtering at an oblique incidence relative to the normal to the substrate. During deposition, the network of primary microstructures is rotated during the deposition step. Many publications have shown that it is thus possible to achieve nano-structuring of the deposited material. With this technique, various elongated shapes can be obtained: zigzag, helix, solid or hollow cylinder with walls, etc. The direction of the elongated structure is either perpendicular or oblique relative to the plane on which it is deposited.

[0125] For the invention, the shape of the secondary nanostructures, their inclination, their density or their distribution (random or not) is not a dimensioning parameter. What matters is the effective index resulting from the arrangement of the nanostructures.

Claims

Claims

1. Optical element (10) transparent to at least one wavelength of use (X0), anti-reflective and super-hydrophobic, comprising a network of sub-wavelength primary microstructures (MSI) made of a first material (Ml) and formed on a substrate (Sub), the primary microstructures having a conical or truncated cone shape, a first width (Ll) and a first height (Hl) such that a ratio between the first width and the first height is less than / 2, and a network pitch (pl) less than the first height (Hl) of said primary microstructure, the network of primary microstructures thus having a structured surface (S), said structured surface being at least partially covered with a plurality of secondary microstructures (MS2) having a nano-pillar or nano-cone shape and being made of a second material (M2) different from the first material,the second material having a hardness strictly greater than the hardness of the first material.,

2. An optical element according to claim 1, wherein the secondary microstructures cover the entire structured surface.

3. An optical element according to any preceding claim, wherein the array of primary microstructures is periodic, in a square or hexagonal mesh.

4. An optical element according to any preceding claim wherein a surface density (DS2) of the secondary microstructures in the coated areas is determined such that an effective index of the plurality of secondary microstructures (neff / f) varies monotonically in a vertical direction perpendicular to the plane of the substrate.

5. An optical element according to any preceding claim wherein a surface density (DS2) of the secondary microstructures in the coated areas is determined such that an effective index of the plurality of secondary microstructures (neff / f) is between the index of air and the index of the substrate (ns).

6. Optical element according to one of the preceding claims in which the second material is chosen from: diamond, alumina, DLC, SiN.

7. An optical element according to one of the preceding claims wherein the second material is boron-doped diamond.

8. Optical element according to one of the preceding claims further comprising a layer (LM2) of second material arranged on the primary structures and on which the secondary microstructures are arranged.

9. An optical element according to any preceding claim wherein the hardness of the second material is greater than or equal to 1.3 times the hardness of the first material, the hardness being measured on the Knoop scale.

10. An optical element according to any preceding claim wherein the wavelength of use is in the visible or near infrared range, and wherein the hardness of the second material is greater than or equal to 3 times the hardness of the first material, the hardness being measured on the Knoop scale.

11. Optical element according to one of claims 1 to 9 in which the wavelength of use is included in the spectral band [3-5 pm] or the spectral band [8-12 pm] and in which the hardness of the second material is greater than or equal to 5 times the hardness of the first material, the hardness being measured on the Knoop scale.

12. An optical element according to any preceding claim wherein the secondary microstructures are arranged in a forest or grass pattern.

13. Method for manufacturing an optical element (10) transparent to at least one wavelength of use (X0), anti-reflective and superhydrophobic, comprising the steps of: • providing a network of primary microstructures (MSI) sub-wavelength in a first material (Ml) and formed on a substrate (Sub), the primary microstructures having a conical or truncated shape, a first width (Ll) and a first height (Hl) such that a ratio between the first width and the first height is less than / 2, and a pitch of the network (pl) less than the first height (Hl) of said primary microstructure, the network of primary microstructures thus having a structured surface (S), • deposit on the structured surface a layer of a second material different from the first material and having a hardness strictly greater than that of the first material, • etching said structured surface by plasma, ionic, chemical etching or a combination of these techniques so that the structured surface is covered at least partially with a plurality of secondary microstructures (MS2) having a shape of nano-pillars or nano-cones.

14. Method according to the preceding claim in which the etching is a reactive ion etching.

15. Method for manufacturing an optical element (10) transparent at a use wavelength (X0), anti-reflective and super-hydrophobic, comprising the steps of: • having a network of primary microstructures (MSI) at sub-wavelength in a first material (Ml) and formed on a substrate (Sub), the primary microstructures having a conical or truncated conical shape, a first width (Ll) and a first height (Hl) such that a ratio between the first width and the first height is less than / 2, and a network pitch (pl) less than the first height (Hl) of said primary microstructure, the network of primary microstructures thus having a structured surface (S), • depositing by evaporation or cathodic sputtering a second material (M2) at an oblique incidence relative to the normal to the substrate so that the structured surface is at least partially covered with a plurality of secondary microstructures (MS2) having the shape of nano-pillars or nano-cones, the network of primary microstructures being in rotation during the deposition step, the second material being different from the first material and having a hardness strictly greater than that of the first material.