Multilayer metasurface structures and fabrication thereof

Abstract

The invention is notably directed to a method of obtaining a multilayer metasurface structure (1). The method comprises forming (S10 – S40) a layer structure (1) including two or more sub-stacks (10) of layers, wherein all of the sub-stacks of layers are stacked on a substrate (11). Each sub-stack (10) is obtained by: depositing (S20) a metal film (12) on a previous dielectric material layer (16) of the layer structure (1) obtained so far; obtaining (S30) an essentially two-dimensional arrangement of nanoparticles (14) through a controlled solid-state dewetting of the deposited metal film (12); and conformally covering (S40) the nanoparticles (14) with a dielectric material layer (15) for the latter to form a dielectric spacer. The method is simple (it does not require any structured substrate, structured support layers, or patterned metal film, and does not require complex lithographic steps), cost-effective, eco-friendly, and makes it possible to obtain ultra-clean nanoparticles. Clean interfaces are obtained, not only between the nanoparticles and the lower support layer (e.g., the dielectric material layer of the lower sub-stack) but also between the nanoparticles and the upper dielectric material, with a high level of conformality. This helps eliminate unwanted scattering centres such as surface defects and internal voids. The above method can be used to easily tune electro-optical properties of the layer structure, e.g., to enhance a visual appearance of a luxury item. The invention is further directed to related multilayer metasurface structures as obtained thanks to the above method.

Description

[0001] MULTILAYER METASURFACE STRUCTURES AND FABRICATION THEREOF

[0002] TECHNICAL FIELD

[0003] The invention relates in general to methods of obtaining multilayer metasurface structures, as well as the resulting structures, i.e., multilayer metasurface structures that can be obtained with such methods. In particular, it is directed to fabrication processes relying on solid-state dewetting of metal layers to obtain two-dimensional arrangements of nanoparticles, where each layer of nanoparticles is subsequently covered, conformally, to form dielectric spacers between two consecutive layers of nanoparticles.

[0004] BACKGROUND

[0005] Advancements in multilayer and hybrid nanolaminate design have led to the development of coatings with outstanding electro-optical properties. Such coatings are poised to transform various industries, offering potential in both decorative and functional applications. They can be used to create colour filters that enhance image quality, precise optical sensors that significantly improve measurement accuracy, as well as durable protective coatings. The latter application is particularly crucial in the watch industry, where innovative coatings are sought to elevate the longevity and resilience of timepieces.

[0006] The use of pigments to create striking and vibrant colours through interference effects has revolutionized the field of decorative coatings and visual applications. One key benefit of employing interference colours is the ability to precisely control and manipulate colour outcomes by adjusting interlayer thickness and chemistry, offering an exceptional level of customization. This technique empowers designers and manufacturers to create bold, unique colours without requiring extensive research or optimization.

[0007] An alternative method for enhancing visual appearances relies on plasmonic nano-colour coatings, which consist of periodically arranged metal nanoparticles and dielectric layers. These coatings derive their colours primarily from selective absorption due to localized surface plasmon resonance and the scattering of unabsorbed light. Additional factors, such as surface light reflection and volume / surface scattering, contribute to the coatings' overall gloss and translucency. The optical properties and colours can be fine-tuned by adjusting nanoparticle size and interparticle distance. Moreover, the colours of plasmonic nanolayer coatings are influenced by optical refractive indices, metal particle layer thickness, and interlayer spacing. This versatile approach offers a high degree of customization as well.

[0008] There are two main approaches to plasmonic nano-colour coatings. The first is the so- called wet coating method, which utilizes paint to create a layer typically tens to hundreds

[0009] P193704PC00 SRA 16.09.25 of micrometres thickness. This technique achieves vivid colours with high chroma. However, it often involves hazardous, environmentally unfriendly chemicals in the fabrication process, posing risks to both health and the environment. The second approach is the dry deposition of multilayer coatings, which presents its own challenges. This method requires precise control of deposition parameters to achieve vivid colours. Additionally, the visual effects can vary unpredictably based on the direction of incident light and viewing angles, which may compromise the consistency of colour presentation.

[0010] Recently, a new technology, using surface plasmon resonance of nanoparticles embedded in an optically transparent dielectric matrix, has been proposed as an alternative for enhancing colours. However, to obtain reasonably high chroma, the films require a high density of nanoparticles, which increases the consumption of precious metals, making it less cost-effective for large-scale production. Moreover, producing well-defined sizes and distributions of nanoparticles using advanced lithography tools is not only costly but also limited to wafer-level or large batch fabrication. As a result, this approach faces significant hurdles in terms of scalability and cost-efficiency for a wide range of applications.

[0011] The following documents are examples of documents illustrating the background art: US20240085598A1, US20210010139A1, US20200156934A1, US20200293001A1,

[0012] US9482798B2, US7486400B2, and US20210263211A1.

[0013] With this in mind, the inventors set themselves the challenge of drastically simplifying processes of fabrication of multilayer structure of embedded nanoparticles.

[0014] SUMMARY

[0015] According to a first aspect, the invention is embodied as a method of obtaining a multilayer metasurface structure. The method revolves around forming a layer structure that includes two or more sub-stacks of layers, where all of the sub-stacks are stacked on a substrate. Each sub-stack of said sub-stacks of layers is obtained by: depositing a metal film on a previous dielectric layer of the layer structure obtained so far; obtaining an essentially two-dimensional arrangement of nanoparticles through a controlled solid-state dewetting of the deposited metal film; and conformally covering the nanoparticles with a dielectric material layer for the latter to form a dielectric spacer.

[0016] The above method has multiple advantages. It is simple, cost-effective, eco-friendly, scalable, and makes it possible to obtain ultra-clean nanoparticles. The latter are well- defined, clearly separated from the lower layer and cleanly integrated into the upper dielectric matrix. That is, clean interfaces are obtained, not only between the nanoparticles and the lower support layer (e.g., the dielectric material layer of the lower sub-stack) but also between the nanoparticles and the upper dielectric material, with a high level of

[0017] P193704PC00 SRA 16.09.25 conformality. This helps eliminate unwanted scattering centres such as surface defects and internal voids. While dewetting during processing of thin films in micro- and nanostructures is mostly undesirable (measures to avoid dewetting are often implemented), here one takes advantage of this phenomenon to drastically simplify the fabrication process of nanoparticles. A dewetting process as contemplated herein is simple to implement and can easily be controlled.

[0018] Remarkably, the proposed method allows independent control over the particle size and distribution during the multilayer fabrication, by adjusting the metal layer thickness and suitably controlling the subsequent dewetting process. Interestingly, the present approach does not require any structured substrate, structured support layers, or pre-patterned metal film (although it can easily be adapted to achieve a templated dewetting). It does not require complex lithographic steps either. The method can be performed in a same place (e.g., a single reactor) and can be applied efficiently to various types of substrates, including large wafers to obtain wafer-scale layer structures.

[0019] Interestingly, this approach can be applied to parts of different substrate materials, which may possibly have curved surfaces. More generally, the versatility of the proposed approach makes it suitable for applications to substrates of various material, shapes, sizes, and geometries. In particular, the present approach can suitably be applied to glasses, lenses, jewellery, and watches.

[0020] As one understands, this approach can for example be used to create and enhance a visual appearance, e.g., of a luxury item. More generally, beyond advantages in terms of visual appearances, the resulting layer structures can have multiple applications. The above fabrication method can notably be used to achieve or enhance certain optical and / or electrical properties of the layer structures, or tailor electrical and / or mechanical properties of the resulting layer structures.

[0021] In embodiments, the nanoparticles are conformally covered with the dielectric material layer by depositing the dielectric material layer on the nanoparticles through atomic layer deposition (ALD). This makes it possible to obtain a conformal dielectric material layer with near-angstrom level thickness resolution. Atomic-scale passivation and high conformality around the nanoparticles help eliminate unwanted scattering centres. In turn, even if the whole layer structure is being annealed (to perform the next dewetting), the embedded nanoparticles remain stable during the next dewetting cycle, thanks to the very high conformality achieved through ALD.

[0022] The dewetting process may simply be controlled by applying heat during a given time. That is, in embodiments, the step of obtaining the essentially two-dimensional arrangement of nanoparticles comprises annealing the layer structure to cause the solid-

[0023] P193704PC00 SRA 16.09.25 state dewetting of the deposited metal film. As a result, the nanoparticles form a disordered, though essentially planar, array of particles in each sub-stack. This planarity improves the conformality of the next dielectric layer. In particular, the annealing process can be controlled to that the nanoparticles have, in each sub-stack, an average areal density of 25 - 350 particles / pm2, while an average in-plane dimension of the nanoparticles is of between 15 and 130 nanometres, preferably of between 30 and 100 nanometres. Such ranges give rise to remarkable electro-optical properties, which can notably be exploited to enhance the visual appearance of the multilayer metasurface structure or tune in-plane vs. transverse electrical conductivities. Other applications may require larger particle sizes. For example, one may want to tune mechanical properties of the multilayer structure 1 by using nanoparticles having average sized ranging from 15 nm to several hundred of nanometres.

[0024] Meanwhile, the transverse (i.e., out-of-plane) dimensions of the nanoparticles will typically be of between 5 to 100 nm, something that will also depend on the thickness of the initially deposited metal film as well as the dewetting process. One will typically try to achieve nanoparticles having an average diameter of between 30 and 100 nm to enhance the visual appearance of the multilayer structure, when using the latter as a coating.

[0025] The thickness of the deposited metal film typically varies between 5 and 100 nanometres. In preferred embodiments, however, the deposited metal film has an average thickness of between 5 and 30 nanometres, preferably of between 10 and 20 nanometres. Such dimensions facilitate subsequent dewetting and allow to obtain particle sizes suitable for a range of applications. The metal film is preferably deposited through sputtering or physical vapour deposition, and more preferably through electron-beam deposition, which makes it possible to achieve clean and ultra-thin films, i.e., uniform films below 5 nm thickness.

[0026] In embodiments, a thickness of the deposited dielectric material layer is at least equal to, through preferably larger than, a maximum transverse dimension of the nanoparticles in the same sub-stack. Still, the thickness of the deposited dielectric material layer is preferably less than 800 nanometres, and preferably less than 600 nm, at least in applications leveraging optical interference effects. In typical applications, the thickness of the deposited dielectric material layer is of between 50 and 300 nanometres, and preferably of between 100 and 200 nanometres. Such intervals are compatible with ALD (and other deposition techniques), while allowing a range of interesting electro-optical properties to be achieved. In particular, thicknesses of between 100 and 200 nanometres can be desired to optimally combine optical interference effects with surface plasmonic effects resulting from nanoparticle sizes as mentioned above.

[0027] P193704PC00 SRA 16.09.25 In embodiments, the substrate is a wafer, preferably a silicon wafer, and the two or more sub-stacks of layers are obtained so as for the layer structure to be formed as a waferscale layer structure, hence an efficient approach for large wafer-scale fabrication of multilayer stacks. Alternatively, other types of substrates can be used. For example, the substrate used may be a 3D-structured component, having a more or less complex geometry.

[0028] In embodiments, the two or more sub-stacks of layers are obtained in a same reactor. Accordingly, there is no need to move the layer structure from one place to the other, midstream, which eases the fabrication process, reduces the costs, and prevents inadvertent damages or deformations of the layer structure. Alternatively, other approaches can be contemplated. For example, the dielectric and metal layer may be deposited in a same reactor, while the annealing is performed in a furnace, outside of the reactor.

[0029] The present approach lends itself well to varying experimental parameters from one process cycle to the other, something that can be exploited to tune the electro-optical properties of the resulting layer structure. For instance, in embodiments, forming the layer structure further comprises varying thicknesses of the metal layers deposited and / or thicknesses of the dielectric material layers, from one of the sub-stacks of layers to another. In addition, the step of forming the layer structure preferably comprises varying compositions of the metal layers deposited and / or compositions of the dielectric material layers, from one of the sub-stacks of layers to another. For example, varying the thicknesses of the dielectric spacers makes it possible to tune the resulting optical interference patterns or obtain more complex optical interference patterns; the dielectric spacers are used as optical spacers in that case.

[0030] Also, in embodiments, forming the layer structure further comprises varying compositions of the metal film deposited, thicknesses of the metal film deposited, and / or experimental conditions under which the solid-state dewetting of the deposited metal film is performed, from one of the sub-stacks of layers to another, so as to change the arrangements, sizes, and / or compositions, of the nanoparticles as obtained after the dewetting process. This, in turn, makes it possible to tune the scattering of light.

[0031] In embodiments, at the step of depositing the metal film, the previous dielectric layer is a dielectric material layer of a lower sub-stack, i.e., the sub-stack that was obtained last. There is no need of intermediate layers; here one takes advantage of the wetting properties of the dielectric layer underneath.

[0032] According to another aspect, the invention is embodied as a multilayer metasurface structure, which is obtainable by a method according to any of the above embodiments.

[0033] P193704PC00 SRA 16.09.25 As a result, the multilayer metasurface structure comprises a layer structure with a substrate and two or more sub-stacks of layers, where all of the layer sub-stacks are stacked on the substrate. Again, different types of substrates can be contemplated (with various materials and shapes), as indicated above. Consistently with the first aspect of the invention, each sub-stack includes an essentially two-dimensional arrangement of nanoparticles and a dielectric spacer. The nanoparticles are solid-state dewetted particles of metal arranged on a lower layer of the layer structure. I.e., the nanoparticles are islands or aggregates, obtained by breakup and agglomeration of the thin metal films, arranged on top of the lower layer, which is preferably a dielectric material layer, e.g., a layer that conformally covers nanoparticles of a lower sub-stack. Each dielectric spacer is a dielectric material layer conformally covering the nanoparticles of the corresponding sub-stack.

[0034] In embodiments, in each sub-stack, the nanoparticles form a disordered, albeit essentially planar, array of particles, which have an average areal density of 25 - 350 particles / pm2. An average in-plane dimension of the nanoparticles is of between 15 and 130 nanometres, preferably of between 30 and 100 nanometres.

[0035] In embodiments, the layer structure includes hybrid layer sub-stacks. I.e., the sub-stacks have nanoparticles of average in-plane sizes, arrangements, and / or material compositions, that substantially differ from one of the sub-stacks to another (e.g., by a factor at least 1.5). Optionally, the layer structure may further include a sub-stack of layers consisting of a non-dewetted metal film covered by a dielectric spacer, which is a dielectric material layer.

[0036] In preferred embodiments, the nanoparticles of the sub-stacks of layers include one or more of Au, Ag, and Al, and the dielectric material layer of each of the sub-stacks of layers is a layer of aluminium oxide.

[0037] In embodiments, the dielectric material layer of each of the sub-stacks of layers has a thickness of between 50 and 300 nanometres, preferably of between 100 and 200 nanometres. Still, the thickness of the dielectric material layer is larger than a maximum transverse dimension of the nanoparticles in the same sub-stack. In addition, the dielectric material layer preferably has a thickness that differs from one of the sub-stacks to another, by a factor of more than 1.3.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. The illustrations are for clarity

[0040] P193704PC00 SRA 16.09.25 in facilitating one skilled in the art in understanding the invention in conjunction with the detailed description. In the drawings:

[0041] FIG. 1 illustrates an iterated process of fabrication of a multilayer metasurface structure according to embodiments of the invention;

[0042] FIG. 2 shows a cross section of a multilayer metasurface structure according to embodiments. The multilayer metasurface structure includes a layer structure with multiple sub-stacks of layers, all stacked on a substrate, wherein the thickness of the dielectric spacers varies from one sub-stack to another. In addition, the nanoparticle arrangements (sizes and distributions) vary from one sub-stack to another as well. One of the sub-stacks includes a non-dewetted metal film;

[0043] FIG. 3 shows a 3D view of a multilayer metasurface structure similar to that of FIG. 2, except that the layer sub-stacks are essentially similar, as in embodiments;

[0044] FIG. 4A - 50 show experimental cross-sectional images of actual multilayer metasurface structures (all according to embodiments) obtained by scanning transmission electron microscopy (STEM);

[0045] FIG. 6 illustrates the specular vs. diffuse reflection of incident light on a multilayer metasurface structure according to embodiments of the invention;

[0046] FIG. 7 shows a reactor that can be used to perform all steps of a fabrication process according to embodiments; and

[0047] FIG. 8 is a flowchart illustrating high-level steps of such a process of fabrication, according to embodiments.

[0048] The accompanying drawings show simplified representations of multilayer metasurface structures according to embodiments, as well as devices or parts thereof, as involved in embodiments. Technical features depicted in the drawings are not necessarily to scale. Similar or functionally similar elements in the figures have been allocated the same numeral references, unless otherwise indicated.

[0049] Fabrication methods and multilayer metasurface structures embodying the present invention will now be described, by way of non-limiting examples.

[0050] DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0051] Referring to FIGS. 1 - 3, and 9, a first aspect of the invention concerns a method of obtaining a multilayer metasurface structure 1, i.e., a nanostructured surface coating comprising multiple stacks of plasmonic metal nanoparticles embedded in dielectric

[0052] P193704PC00 SRA 16.09.25 spacers. This method and its variants are collectively referred to as the "present methods" in this document. All references Sn refer to methods steps of the flowchart of FIG. 8, while numeral references pertain to materials, devices, components, and other physical concepts as involved in embodiments of the present invention.

[0053] The method revolves around forming a layer structure 1 (steps S10 - S40 in FIG. 8), which includes two or more sub-stacks 10 of layers. Note, both this layer structure and the resulting multilayer metasurface structure are denoted by the same numeral reference ("1"), because the multilayer metasurface structure may essentially consist of the layer structure 1, in embodiments. In variants, the multilayer metasurface structure may further include additional elements, such as ohmic contacts, lateral structures, etc. As seen in FIGS. 2 or 3. The various layer sub-stacks 10 are stacked (i.e., superimposed or cascaded) on the substrate 11, so as to form a layer structure 1 of superimposed layers. As illustrated in FIG. 1, each sub-stack 10 is obtained by depositing S20 a metal film 12, forming S30 nanoparticles from this film, and then coating S40 the nanoparticles with a dielectric material layer 15. This process is repeated, as necessary to form multiple sub-stacks, i.e., cascaded metasurfaces.

[0054] In detail, each sub-stack is formed by depositing (step S20) a metal film 12 on a previous dielectric layer 16 of the layer structure 1 obtained so far. This previous layer 16 will preferably be a dielectric material layer 15 of the sub-stack 10 obtained last, as suggested by the cycle shown in FIG. 1, see also FIGS. 2 and 3. The metal films 12 of the multiple sub-stacks 10 may for instance contain one or more of Ag, Au, Cu, Pt, Pd, Ni, Fe, AuAg, CuPt, CuPd, AuPt, and AuAgPd. The level of purity of the metal film will preferably be very high (e.g., 99.99% or more). Preferably, the metal layers include Ag or Au. The composition of the metal layer 12 can change from one sub-stack to the next. Thus, two consecutive layers of nanoparticles 14 may contain different metals, or mixtures thereof. For instance, in embodiments, the layer sub-stacks 10 include metal layers of alternating elements, e.g., first Au, then Ag, then Au, etc. In addition, the size and arrangements of nanoparticles may vary from sub-stack to sub-stack, as discussed later in detail.

[0055] Next, an essentially two-dimensional (2D) arrangement of nanoparticles 14 is obtained S30 through a controlled process of solid-state dewetting of the metal film 12 deposited. In the present document, solid-state dewetting refers to a phenomenon, according to which solid films of metal deposited on a dielectric layer may, under certain conditions, be metastable or unstable enough, so that they will break up and agglomerate locally in multiple places to form islands. This process is driven by surface energy and interface energy minimization. It can occur through surface diffusion and interface diffusion; the driving force lowers the surface energy and the interface energy. This phenomenon can occur already well below the melting temperature of the film, especially if the latter is very

[0056] P193704PC00 SRA 16.09.25 thin. The experimental conditions (film thickness, materials, temperature, atmosphere composition, etc.) determine the dewetting process. In the present context, the nanoparticles 14 are obtained thanks to the dewetting of the metal film 12 previously deposited, such that the resulting nanoparticles 14 are essentially arranged in a plane, on top of the previous dielectric layer.

[0057] The dewetting mechanism must be controlled (e.g., by controlling the temperature applied to the layer structure and / or other experimental parameters, such as the surrounding gas pressure, as well as the time duration of the dewetting process). In typical applications, the main control parameter will be temperature; the layer structure is annealed for a given time. For example, the temperature applied to an Au metal film will typically be between 200 °C and 900 °C, depending on the film thickness. In addition, the optimal temperature depends on the metal(s) chosen and the interface energies involved. Still, depending on the materials used and the metal film thickness, it may not always be necessary to apply temperature to the layer structure, in which case other control parameters may be relied on (e.g., surrounding gas pressure). As a result of the controlled dewetting process, the nanoparticle layer forms an essentially 2D arrangement of particles, which is sometimes referred to as a nanoparticle layer in the following.

[0058] Finally, the nanoparticles 14 are conformally covered S40 with a dielectric material layer 15, e.g., an oxide or nitride layer, such as SiO?, AI2O3, orTiN. The dielectric material layer forms a dielectric spacer, onto which a new layer can be deposited. In embodiments, the conformal dielectric layer 15 obtained last serves as a support layer for the next metal film 12, which is directly deposited on this dielectric layer 15. Alternatively, an intermediate layer is deposited between the conformal dielectric layer 15 obtained last and the next metal layer 12, if necessary, to help the dewetting process. Preferred, however, is to directly deposit the next metal layer on the preceding dielectric spacer, if only for simplicity. In general, wetting properties of dielectric material layers with respect to thin metal film favourably impact the dewetting process. The dielectric material layer 15 is preferably deposited by atomic layer deposition (ALD), to ensure an optimal conformality.

[0059] The above method has multiple advantages. It is simple, cost-effective, eco-friendly, scalable, and makes it possible to obtain ultra-clean nanoparticles 14. The latter are well- defined, clearly separated from the lower layer and cleanly integrated into the upper dielectric matrix 15. That is, clean interfaces are obtained, not only between the nanoparticles and the lower support layer 16 (e.g., the dielectric material layer 15 of the lower sub-stack) but also between the nanoparticles 14 and the upper dielectric material layer 15, with a high level of conformality. This helps eliminate unwanted scattering centres such as surface defects and internal voids.

[0060] P193704PC00 SRA 16.09.25 While dewetting during processing of thin films in micro- and nanostructures is mostly undesirable (measures to avoid dewetting are often implemented), here one takes advantage of this phenomenon to drastically simplify the fabrication process of nanoparticles. A dewetting process as contemplated herein is simple to implement and can easily be controlled. Remarkably, the proposed method allow independent control over the particle size and distribution during the multilayer fabrication, by controlling the metal layer thickness and the subsequent dewetting process.

[0061] Interestingly, the present approach does not require complex lithographic steps. It does not require any structured substrate, structured support layers, or pre-patterned metal film either. For instance, while template-based approaches to dewetting are known, which rely on patterned surface topographies or pre-patterned films to create ordered arrays of particles or complex patterns of partially dewetted structures, the present methods are much preferably free of such constraints. I.e., they do preferably not rely on patterned support surfaces or patterned metal films. That being said, use can possibly be made of templated surfaces, if necessary, to better control the size distribution and spatial arrangement of nanoparticles. Last but not least, all operations can advantageously be performed in a same reactor, in the interest of time efficiency and costs. The method can be performed in a same place (e.g., a single reactor) and can be applied efficiently to large wafers to obtain wafer-scale layer structures.

[0062] One may want to exploit optical properties of the multilayer metasurface structure obtained. This multilayer metasurface structure can for example be used as a coating to create and enhance a visual appearance, e.g., of a luxury item (the dielectric spacers are also used as optical spacers in that case). The thicknesses of the dielectric spacers 15 impacts the optical interference patterns formed upon irradiating the resulting structure 1. The dielectric material layer 15 will typically have a thickness of between 50 and 300 nanometres, preferably of between 100 and 200 nanometres. Another important factor for the resulting electro-optical properties are the metal nanoparticle sizes and arrangements. Interestingly, the coating 1 may be formed a priori or a posteriori. When formed a priori, the coating can be latter re-processed to coat any object (e.g., glass rim, jewel, car parts, etc.) of interest. If necessary, the substrate 11 can be dissolved in order to obtain a freestanding layer sub-stack, which can then be applied onto an object of interest. When formed a posteriori, the coating is grown onto an outer surface of the object.

[0063] The above fabrication method can be used to achieve or enhance certain optical, electrical, and / or mechanical properties. Beyond its advantages in terms of optical properties (e.g., for decorative applications), the above method and the resulting multilayer metasurface structure can also be useful to exploit particular electrical properties of the resulting layer structure. For example, ZnO films with Ag nanoparticles can tremendously increase the

[0064] P193704PC00 SRA 16.09.25 conductivity. More generally, the present approach can be used to create new colour filters, optical sensors, and protective coatings, e.g., for the watch industry, as noted in the background section.

[0065] All this is now described in detail, in reference to particular embodiments of the invention. To start with, in preferred embodiments, the nanoparticles 14 are conformally covered S40 with a dielectric material layer 15 by depositing the latter on the nanoparticles through atomic layer deposition (ALD). In principle, various chemical vapour deposition techniques can be contemplated. However, ALD makes it possible to obtain a conformal dielectric material layer with near-angstrom level thickness resolution. Atomic-scale passivation and high conformality around the nanoparticles virtually eliminate scattering centres. In turn, even if the whole layer structure is being annealed (to perform the next dewetting), the embedded nanoparticles remain stable during the next dewetting cycle, thanks to the very high conformality achieved through ALD. In this regard, different solutions can be contemplated to heat the layer structure. For example, ex-situ heating (in a suitable furnace) can be used for noble metals, without risking oxidation. A preferred variant, however, is to perform all steps in a same reactor. One may for example use a high- temperature stage with a sample holder including a heating element that is heated by conduction (Joule heating).

[0066] Depending on the materials used, the thickness of the deposited films, and the ambient temperature, the solid films deposited may well be metastable or unstable, such that they will dewet, i.e., agglomerate to form small metal islands. The dewetting process may notably be controlled by applying heat, during a given time. I.e., in embodiments, step S30 includes annealing the layer structure 1. That is, the temperature of the layer structure is increased, e.g., by heating the layer structure 1, either indirectly through the atmosphere (i.e., the gas surrounding the layer structure) or, preferably, directly, through a heater. The annealing step triggers or accelerates the solid-state dewetting of the deposited metal film 12, as illustrated in FIG. 1.

[0067] Namely, the annealing step causes (or substantially increases) surface tensions (denoted by small black arrows in FIG. 1) between the lower layer 16 and the metal film 12, which, in turn, causes the metal layer to agglomerate 14i and eventually form the nanoparticles 14. As a result, nanoparticles form during each cycle, i.e., in each sub-stack 10. Because the nanoparticles agglomerate on a dielectric layer, which can be made essentially planar, the resulting nanoparticles 14 may form an essentially planar array of particles, which is nonetheless disordered in-plane. This can be seen in FIGS. 4A - 5C, which are images obtained by scanning transmission electron microscopy (STEM).

[0068] The time and duration of the annealing step are determined on a case-by-case basis and can be optimized using trial and error. Optimal annealing parameters depend on a number

[0069] P193704PC00 SRA 16.09.25 of factors, e.g., the materials involved, their dimensions, etc. Such parameters can be set so that the resulting nanoparticles have an average areal density of 25 - 350 particles / pm2, the initial thickness of the metal film permitting. Meanwhile their average in-plane dimension is of between 15 and 130 nanometres, preferably of between 30 and 100 nanometres. The above ranges give rise to remarkable electro-optical properties, which can notably be exploited to enhance the visual appearance of the multilayer metasurface structure 1 or tune in-plane vs. transverse electrical conductivities.

[0070] The sizes and densities of the nanoparticles can be adjusted depending on the intended application. In practice, the sizes and densities also depend on a number of parameters, e.g., initial thickness of the metal film and crystalline state of the substrate (single crystal, polycrystalline, or amorphous), annealing environment and heating rate. Some of these parameters may have to be adjusted in accordance with the desired properties of the nanoparticles. For example, the nanoparticle sizes can be tailored by changing the film thickness before dewetting, as discussed below.

[0071] In particular, the following experimental parameters can be adjusted: (i) the choice of deposited materials (metal layer and dielectric layer); (ii) the thicknesses of such materials; and (iii) the duration and intensity of the annealing step, as well as the heating rate. The materials chosen and their thicknesses determine the resulting electro-optical properties, such as the absorption coefficient. The thicknesses of the dielectric layers impacts the optical interferences and electrical conductivity (in-plane and out-of-plane). So does the metal layer thickness, which impacts the nanoparticle sizes. The sizes and distributions of the nanoparticles (which determine the plasmonic effects) will further depend on the annealing parameters (temperature and time duration). All such parameters can be jointly optimized. In particular, the annealing parameters will depend on the materials chosen and their thicknesses; they normally scale with the thickness. Typically, the layer structure is heated to a temperature of between 200 °C to 900 °C, during minutes to hours, depending on the environment. Optimal parameter values can be roughly determined based on knowledge accumulated in materials science and refined by trial and error.

[0072] As noted above, optimal annealing parameters notably depend on the thickness of the deposited metal film 12. In embodiments, the deposited metal film 12 has an average thickness of between 5 and 30 nanometres, and preferably of between 10 and 20 nanometres. Such dimensions facilitate subsequent dewetting and allow to obtain particle sizes suitable for a range of applications. The metal film 12 can for instance be deposited through sputtering or physical vapour deposition (e.g., thermal evaporation, electronbeam lithography, etc.). More preferably, it is deposited through electron-beam lithography. However, the optimal deposition technique depends on several parameters,

[0073] P193704PC00 SRA 16.09.25 starting with the materials used. Sputtering was used for the multilayer metasurface structure shown in the accompanying STEM images.

[0074] Another experimental parameter is the thickness of the dielectric spacers 15. The thickness of the deposited dielectric material layer will normally be at least equal to, and preferably larger than, the average transverse dimension of the nanoparticles. Meanwhile, the thickness of the deposited dielectric material layer will preferably be less than 800 nanometres, at least in applications leveraging optical interference effects. In practice, the deposited dielectric material layer 15 will typically have a thickness of between 50 and 300 nanometres, and preferably of between 100 and 200 nanometres, as in the examples of FIGS. 4A - 5C. Such intervals are compatible with ALD (and other deposition techniques), while allowing a range of electro-optical properties to be achieved.

[0075] The layer structures shown in FIGS. 4A - 5C were obtained according to the following experimental parameters. Sputtering parameters (for 99.99% pure gold): 50mA, 400V, 20KW power, base pressure of IxlO-7mbar, deposition pressure of 5xl0-3mbar, deposition rate of 8 nm / minute. ALD parameters: Precursors Trimethylaluminum (TMA, AI(CH3)3) and Water (H2O), deposition temperature of 250 °C, Growth rate of 0.1 nm per cycle. In this example, the ALD process can be carried out at a temperature ranging from room temperature to 400 °C. However, it is preferably performed at a temperature above 200 °C as this reduces the blistering of the films (less effusion of hydrogenous molecules or hydrogen atoms) after the high temperature annealing.

[0076] In FIGS. 40 to 5B, the thickness of the dielectric material layers 15 is essentially constant from one sub-stack to another. Such dielectric material layers are thus denoted by the same numeral reference 15. However, the dielectric spacers do not necessarily have the same thickness. For example, in FIG. 5C, the thicknesses of the dielectric material layers 15, 15a differ substantially from one sub-stacks to another. Namely, the lower layer 15 has a thickness of approximately 200 nm, while the upper layer 15a has a thickness of approximately 150 nm. That is, the thicknesses of the two layers 15, 15a differ by a factor of more than 1.3. Having dielectric spacers of different thicknesses makes it possible to create more complex interference optical patterns in the visible spectrum.

[0077] In embodiments, the substrate is a wafer 11, e.g., a silicon wafer 11. Interestingly, the present fabrication methods scales well, laterally, and can be performed on a wafer scale. That is, the layer sub-stacks 10 can be grown so that the layer structure 1 eventually obtained is a wafer-scale layer structure 1, hence an efficient approach for large waferscale fabrication of multilayer stacks. That being said, it is worth reminding that the present approach works very well with other types of substrates, including curved substrates (e.g., optical lenses, 3D parts, etc.). Also, the present approach may be applied to multiple substrates, e.g., multiple wafers, where any or all of the substrates may have

[0078] P193704PC00 SRA 16.09.25 curved surfaces or another geometry. More generally, the versatility of the proposed approach makes it suitable for applications to substrates of various material, shapes, sizes, and geometries. In particular, the present approach can suitably be applied to glasses, lenses, jewellery, and watches.

[0079] For example, disordered optical metasurfaces (MSs) can be cascaded as follows. A metal film (Au) is deposited through metal sputtering on a 4" Si wafer. Annealing the layer structure causes surface tensions that induce the formation 14i of Au nanoparticles, see FIG. 1. The average size, in-plane, of the resulting Au nanoparticles 14 is 50 nm. The nanoparticles are then coated through metal oxide ALD within the same reactor, so as to achieve 150 nm thick AI2O3 dielectric spacers. Consecutive deposition and annealing result in cascading MS sub-stacks. The resulting layer structure 1 has a unique visual appearance, producing angularly separated specular and diffused reflected light, as schematically illustrated in FIG. 6. Eventually, the layer structure 1 can be further processed, e.g., cut and diced, for any desired application. If necessary, the substrate 11 is dissolved, before applying the residual metasurface structure 1 to the surface of a given item or device.

[0080] In the above example, the layer sub-stacks have substantially the same thickness and compositions. However, one may want to vary these parameters from one process cycle to another and, therefore, from one sub-stack to another. For example, one may change the thicknesses of the metal layers 12 deposited. In addition, or in variants, one may further change thicknesses of the dielectric material layers 15, 15a. In addition, one may also change the compositions of the metal layers and / or the dielectric material layers. Doing so results in varying the particle sizes and / or the thicknesses of the dielectric spacers from one sub-stack 10, 10a to another 10a, 10b, as illustrated in FIGS. 2 and 6, something that can be exploited to tune electro-optical properties of the layer structure.

[0081] In particular, one may vary compositions of the metal film 12, thicknesses of the metal film 12, and / or experimental conditions under which the solid-state dewetting of the deposited metal film 12 is performed, from one process cycle to the next. What results is a hybrid structure 1, containing sub-stacks with different arrangements, sizes, and / or compositions of the nanoparticles. This, in turn, impacts the electro-optical properties of the layer structure 1. For example, two consecutive sub-stacks may differ in terms of composition and / or in-plane size of the nanoparticles, e.g., by a factor of at least two, as assumed in FIG. 2 (compare nanoparticles in the sub-stacks 10 and 10b).

[0082] Even more so, one or more of the layer sub-stacks 10 - 10b may include a uniform metal film 12u, i.e., a film not dewetted, as assumed in FIG. 2. In that case, no annealing is performed, meaning that step S30 is skipped during that process cycle. That is, at least one of the process cycles consists of depositing S20 a metal film 12 on a lower layer of

[0083] P193704PC00 SRA 16.09.25 the layer structure 1 obtained so far and directly covering S40 the metal film with a dielectric material layer 15 for the latter to form a dielectric spacer 15u.

[0084] In preferred embodiments, the layer structure 1 is obtained in a same reactor, such that there is no need to move the layer structure 1 from one place to the other, midstream. FIG. 7 shows an example of such a reactor, disclosed in WO2024132366A1 (FIG. 3). This reactor can be used to perform all necessary steps.

[0085] Another aspect of the invention is now described in reference to FIGS. 2 - 5C. It concerns a multilayer metasurface structure 1, which can be obtained according to a method as described above in reference to the first aspect of the invention, i.e., a process involving solid-state dewetting. As a result, the layer structure 1 includes a substrate 11 and two or more sub-stacks 10 of layers, all stacked on the substrate 11. Each sub-stack 10 includes an essentially two-dimensional arrangement of nanoparticles 14 and a dielectric spacer 15. The nanoparticles are solid-state dewetted particles of metal, which are arranged on a lower layer of the layer structure. The dielectric spacer is a dielectric material layer 15 that conformally covers the nanoparticles 14. The lower layer is preferably a dielectric material layer 15 of the lower dielectric spacer, i.e., the spacer that conformally covers nanoparticles 14 of the lower sub-stack 10. Thus, the arrangement of nanoparticles 14 in the next sub-stack is directly formed on the dielectric material layer 15 obtained last.

[0086] The dewetted particles obtained have easily recognizable shapes and arrangements, see FIGS. 4B - 5C. In particular, the nanoparticles have a form factor. The eccentricity of the best-fit ellipses in the STEM cross sectional images is typically close to 0.75, with a relatively small standard dispersion (approximately 0.1). Also, their in-plane dimension is typically larger than their transverse dimension. Furthermore, the present multilayer metasurface structures 1 are characterized by a high degree of conformality. In particular, no voids are visible in the STEM images between the nanoparticles and the corresponding dielectric layer. The conformality level corresponding to the average size of the voids left after conformal deposition of the dielectric layer is typically less than 10 angstroms, preferably less than 5 angstroms, or even 3 angstroms. In preferred embodiments, the residual voids are on the order of the angstrom. To summarize, the present structures 1 will preferably involve conformal layers, void-free, and uniform films on high-aspect-ratio surfaces with atomic precision.

[0087] Because the supporting layer 16 (e.g., the dielectric layer 15 of a previous sub-stack) may be made relatively flat, the nanoparticles 14 may form an essentially planar array of particles, although their in-plane arrangement is disordered. As noted earlier, their average areal density will preferably be of between 25 - 350 particles / pm2, while their average in-

[0088] P193704PC00 SRA 16.09.25 plane dimension will preferably be of between 15 and 130 nanometres, and more preferably of between 30 and 100 nanometres. For instance, the nanoparticles 14 may have an in-plane diameter of approximately 50 nanometres, as in the example provided earlier.

[0089] Moreover, in embodiments, the layer sub-stacks 10, 10a, 10b have nanoparticles 14 of average in-plane sizes, arrangements, and / or material compositions, that substantially differ from one sub-stack to another. Even, the layer structure 1 may include a layer substack consisting of a non-dewetted metal film 12u covered by a dielectric spacer 15u, which again is a dielectric material layer 15, as shown in FIG. 2. For example, the sizes of the particles may vary by a factor of at least 1.5 or 2. That is, the layer structure 1 may come as a hybrid structure, with alternating nanoparticle layers or hybrid layers of nanoparticles and non-dewetted films embedded in dielectric layers 15, 15u.

[0090] As noted earlier, the dielectric material layer 15 of each sub-stack 10 will preferably have a thickness of between 50 and 300 nanometres, and more preferably of between 100 and 200 nanometres. Optionally, the thickness of the dielectric material layer differs from one sub-stack to another, by a factor of more than 1.3, which makes it possible to obtain complex optical interference patterns. In preferred embodiments, the nanoparticles 14 of the sub-stacks 10 include one or more of Au, Ag, and Al, while the dielectric material layer 15 of each of the layer sub-stacks is a layer of aluminium oxide.

[0091] While the present invention has been described with reference to a limited number of embodiments, variants, and the accompanying drawings, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the present invention. In particular, a feature (product-like or method-like) recited in a given embodiment, variant or shown in a drawing may be combined with or replace another feature in another embodiment, variant or drawing, without departing from the scope of the present invention. Various combinations of the features described in respect of any of the above embodiments or variants may accordingly be contemplated, that remain within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention is not limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. Many other variants than explicitly touched above can be contemplated. For example, other materials could be used for all layers 12, 15 of interest, which may also have different thicknesses.

[0092] P193704PC00 SRA 16.09.25

Claims

CLAIMS1. A method of obtaining a multilayer metasurface structure (1), wherein the method comprises forming (S10 - S40) a layer structure (1) including two or more sub-stacks (10) of layers, wherein all of the sub-stacks of layers are stacked on a substrate (11), and wherein each sub-stack (10) of said sub-stacks of layers is obtained by: depositing (S20) a metal film (12) on a previous dielectric layer (16) of the layer structure (1) obtained so far; obtaining (S30) an essentially two-dimensional arrangement of nanoparticles (14) through a controlled solid-state dewetting of the deposited metal film (12); and conformally covering (S40) the nanoparticles (14) with a dielectric material layer (15) for the latter to form a dielectric spacer.

2. The method according to claim 1, wherein the nanoparticles (14) are conformally covered (S40) with the dielectric material layer by depositing the dielectric material layer (15) on the nanoparticles through atomic layer deposition.

3. The method according to claim 1 or 2, wherein obtaining (S30) the essentially two-dimensional arrangement of nanoparticles comprises annealing the layer structure (1) to cause the solid-state dewetting of the deposited metal film (12), so that, in said each sub-stack (10), the nanoparticles (14) form a disordered, essentially planar array of particles, which have an average areal density of 25 - 350 particles / pm2, and an average in-plane dimension of the nanoparticles (14) is of between 15 and 130 nanometres, preferably of between 30 and 100 nanometres.

4. The method according to any one of claims 1 to 3, wherein the deposited metal film (12) has an average thickness of between 5 and 30 nanometres, preferably of between 10 and 20 nanometres, and the metal film (12) is preferably deposited through sputtering or physical vapour deposition, and more preferably through electron-beam deposition.

5. The method according to any one of claims 1 to 4, whereinP193704PC00 SRA 16.09.25the deposited dielectric material layer (15) has a thickness of between 50 and 300 nanometres, preferably of between 100 and 200 nanometres, the thickness of the deposited dielectric material layer being larger than a maximum transverse dimension of the nanoparticles in said each sub-stack.

6. The method according to any one of claims 1 to 5, wherein the substrate is a wafer (11), preferably a silicon wafer (11), and the two or more sub-stacks (10) of layers are obtained so as for the layer structure (1) to be formed as a wafer-scale layer structure (1).

7. The method according to any one of claims 1 to 6, wherein the two or more sub-stacks (10) of layers are obtained in a same reactor (100).

8. The method according to any one of claims 1 to 7, wherein forming the layer structure (1) further comprises varying, from one of the substacks of layers to another, thicknesses of the metal layers deposited and / or thicknesses of the dielectric material layers, and, preferably, compositions of the metal layers and / or compositions of the dielectric material layers.

9. The method according to any one of claims 1 to 8, wherein forming the layer structure (1) further comprises varying compositions of the metal film (12) deposited, thicknesses of the metal film (12) deposited, and / or experimental conditions under which the solid-state dewetting of the deposited metal film (12) is performed, from one of the sub-stacks of layers to another.

10. The method according to any one of claims 1 to 9, wherein, at depositing (S20) the metal film (12), said previous dielectric layer (16) is a dielectric material layer (15) of one of the sub-stacks of layers obtained last.

11. A multilayer metasurface structure (1) obtainable by the method according to any one of claims 1 to 10, wherein the multilayer metasurface structure comprises a layer structure (1) with: a substrate (11), andP193704PC00 SRA 16.09.25two or more sub-stacks (10) of layers, wherein all of the sub-stacks of layers are stacked on the substrate (11) and each sub-stack (10) of said sub-stacks of layers includes: an essentially two-dimensional arrangement of nanoparticles (14), which are solid-state dewetted particles of metal arranged on a lower layer of the layer structure; and a dielectric spacer, which is a dielectric material layer (15) conformally covering the nanoparticles (14), wherein, said lower layer is preferably a dielectric material layer (15), which, more preferably, is conformally covering nanoparticles (14) of a lower stack of said sub-stacks of layers.

12. The multilayer metasurface structure (1) according to claim 11, wherein, in said each sub-stack (10), the nanoparticles (14) form a disordered, essentially planar array of particles, which have an average areal density of 25 - 350 particles / pm2, and an average in-plane dimension of the nanoparticles (14) is of between 15 and 130 nanometres, preferably of between 30 and 100 nanometres.

13. The multilayer metasurface structure (1) according to claim 11 or 12, wherein the sub-stacks of layers of the layer structure (1) have nanoparticles of average inplane sizes, arrangements, and / or material compositions, that substantially differ from one of the sub-stacks of layers to another, and the layer structure optionally includes a sub-stack of layers consisting of a nondewetted metal film covered by a dielectric spacer, which is a dielectric material layer (15).

14. The multilayer metasurface structure (1) according to any one of claims 11 to 13, wherein the nanoparticles (14) of the sub-stacks (10) of layers include one or more of Au, Ag, and Al, and the dielectric material layer (15) of each of the sub-stacks of layers is a layer of aluminium oxide.

15. The multilayer metasurface structure (1) according to any one of claims 11 to 13, wherein the dielectric material layer (15) of each of the sub-stacks (10) of layers has a thickness of between 50 and 300 nanometres, preferably of between 100 and 200P193704PC00 SRA 16.09.2520 nanometres, the thickness of the deposited dielectric material layer being larger than a maximum transverse dimension of the nanoparticles in said each sub-stack, and the dielectric material layer (15) preferably has a thickness that differs from one of the sub-stacks (10) of layers to another, by a factor of more than 1.3.P193704PC00 SRA 16.09.25

Images