Security element comprising nanostructures

Abstract

The invention relates to a security element for objects of value or documents of value (2), which security element has, on a substrate (20), a sub-wavelength grating structure (24) which exhibits a colour produced on the basis of plasmon resonance in a perpendicular plan view (8), wherein a multiplicity of sections (16, 17) which cannot be resolved with the naked eye and in each of which the sub-wavelength grating structure (24) is uniformly formed are formed, wherein the sub-wavelength grating structures (24) between the sections (16, 17) differ in at least one structure parameter influencing light refraction, and the multiplicity of sections which cannot be resolved with the naked eye cover a surface area (6, 32) of the security element (4), wherein the sections (16, 17) in the surface area (6, 32) differ in the at least one structure parameter such that an observer does not perceive any rotational-angle-dependent refraction effect in the surface area (6) covered by the sections (16, 17) when the security element (4) is tilted about at least one tilt axis (10) in a substrate plane.

Description

[0001] S i c h e r h e i t s e l e m e n t m i t N a n o s t r u k t u r e n The invention relates to a security element for valuables or valuable documents, comprising a subwavelength grating structure on a substrate that exhibits color through light diffraction and resonance effects, wherein a plurality of partial regions are formed that are not resolvable to the naked eye, in each of which the subwavelength grating structure is uniformly formed, wherein the subwavelength grating structures differ between the partial regions in at least one structural parameter that influences light diffraction. The invention further relates to a method for producing such a security element. Since the 1980s, holographic gratings with a period typically between 600 nm and 1500 nm have been used as security features in banknotes and identity cards. For decades, they have shaped the visual appearance of these documents and ensured their forgery security.The following aspects are relevant to the term "diffraction structures": Diffraction is the deflection of waves by an obstacle. Through diffraction, a wave can propagate into regions of space that would be blocked if it traveled straight through the obstacle. Any type of physical wave can exhibit diffraction. Diffraction occurs when new waves are created along a wavefront according to the Huygens-Fresnel principle. These can lead to interference phenomena through superposition. One example is diffraction at a periodic grating (in the first or higher order). Periodic structures can be used for this purpose. One- or two-dimensional periodic structures ("hologram gratings") with periods from approximately 400 nm to approximately 5 μm typically display colorful "rainbow colors." Special optically variable effects can be created by varying, for example, the grating period and / or azimuth angle.The parameters can be varied continuously or at least in small steps (e.g., to achieve pumping effects). Even larger structures exhibit diffraction effects, although in practice these can only be observed with largely directed illumination. In contrast to gratings with a symmetrical profile (sine-wave gratings, rectangular gratings), blaze gratings have an asymmetrical profile (particularly sawtooth profiles). This allows, for example, more intensity to be diffracted into the +1st diffraction order than into the -1st diffraction order, allowing asymmetric images to be created. Aperiodic diffraction structures act as "matte structures" and have similar structure sizes to hologram gratings, but are irregularly arranged, so that an overlay of light of different wavelengths in the diffracted light leads to a "whitening".The structures can diffract practically isotropically in all directions or only in certain preferred directions. If different structures with different preferred directions are used, achromatic drift effects, for example, can be achieved. Starting with hologram gratings, such structures can be created by "shaking" the grating lines, for example by varying the line spacing or azimuth angles of individual grating lines. In recent years, however, the forgery security of embossed holograms, which are based on the physical effect of diffraction, has noticeably decreased due to the increasing availability of equipment for their production, so that they can hardly be used anymore for the primary protection of valuable documents. Instead of holograms, security features based on the use of non-diffracting microstructures, such as micromirrors or microlenses, are therefore preferred.Such features are significantly harder to counterfeit and give the value documents equipped with them an unusual and novel appearance, combined with a high level of dynamism that quickly attracts attention. The use of color-shift coatings, which give the viewer a color impression dependent on the tilt angle, is highly effective in this context and further increases counterfeit security. Coatings with interference color filters (color shift) create color through the multiple reflection of light in a (thin) layer structure. The interference condition is only met for one wavelength; all others have destructive interference. This results in color generation. With an increasing number of layers, the color becomes spectrally "purer," i.e., the interference peak becomes narrower. Three-layer structures with an absorber, dielectric, and reflector are typical, e.g., absorber Cr (approx.5 nm), dielectric SiO2 (200-500 nm), reflector Al (approx. 50 nm). Micromirrors are facets that reflect incident light essentially according to the laws of ray optics. They have dimensions that are significantly (approx. factor 10) larger than the light wavelength, especially dimensions from about 5 μm. Their reflection behavior is therefore largely achromatic. The dimensions are preferably a maximum of 100 μm, particularly preferably a maximum of 40 μm. The micromirrors can be arranged periodically ("sawtooth grating") or aperiodically. Fresnel structures can also consist entirely or partially of micromirror structures or be approximated by micromirrors. Fresnel structures are created from arbitrarily shaped surfaces by cuts of constant height. For example, micromirrors can also have an arcuate cross-sectional shape and a three-dimensional shape.Micromirrors are often rectangular, but honeycomb (hexagonal) or arbitrarily shaped micromirrors (“mosaic”) are also possible. The surfaces of the micromirrors are preferably flat with a defined inclination. At least the most steeply inclined micromirrors have a profile height that is significantly greater than the wavelength (> 1 μm). The inclinations of the steepest micromirrors are typically in the range of 20°, and in special designs even in the range of 10°. The reflectance or color of the reflected light can be determined by the coating, which can be a simple surface-conformal metallization such as aluminum, a high-index coating such as ZnS, or a multilayer color-shift structure. The color and / or reflectance can also be adjusted by superimposed subwavelength structures.If micromirrors are not coated with a reflective coating, but are at least semi-transparent, effects similar to the mirror effect in reflected light can be observed due to the prism effect in transmitted light. To achieve the required light refraction, the structures must not be embedded or must be located at an interface with a material with a significantly different refractive index. In parallel with microstructures, optically variable security features have been developed for many years. These are based on the use of nanostructures with typical sizes in the sub-wavelength range, i.e., smaller than 400 nm. One- or two-dimensionally structured nanostructures are used in the form of regular sub-wavelength gratings or in the form of irregular structures, e.g., elevations or depressions irregularly arranged on a base surface.Subwavelength structures are so small that no first- or higher-order diffraction effects occur under normal light incidence. The structure sizes are smaller than the wavelength of the light. Different effects are used in different structures: Subwavelength gratings (also referred to in the literature as "zeroth-order gratings") are regular one- or two-dimensional gratings with periods in the subwavelength range. Subwavelength gratings with a metallic coating use plasmon effects to generate colors. In gratings with a dielectric (particularly high-index) coating, however, colors are created by a different effect, namely the resonant excitation of polaritons. The profile shape can be rectangular or sinusoidal, for example. When illuminated and viewed at shallow angles, first-order diffraction can also occur.Moth-eye structures are subwavelength structures that allow a nearly reflection-free transition of light from one medium to another. In dielectric materials, such structures act as an "anti-reflective layer." The structures can be arranged regularly (periodically) or irregularly and can have a tapered cross-section or, for example, as binary rectangular structures. If the moth-eye structures are metallically vapor-deposited, they appear very dark and, in particular, black. In nanostructures regularly arranged in a one- or two-dimensional lattice, resonance effects, such as plasmon excitations, usually lead to color phenomena, since the transmission, reflection, and absorption of the light incident on the grating depend spectrally on parameters that characterize the subwavelength grating, such as period, depth, profile shape, type of coating, and layer thickness.In particular, the period can be used to adjust a specific color impression. The colors generated in this way and perceived by an observer are generally less brilliant and optically variable than the color phenomena caused by diffraction at a holographic grating; rather, they are more pastel-colored and can be observed over a larger angular range without any significant color change. Viewed without a polarizer, the color impression of subwavelength gratings based on plasmon resonance is typically very similar to almost identical, even when the rotation angle in the structural plane is changed, i.e., independent of the azimuth angle. These properties make subwavelength structures particularly interesting for security features in which they are combined with microstructures.Subwavelength structures here are structures with dimensions YRQ^^^^^^QP^YHUVWDQGHQ^^'LH^*LWWHUSHULRGHQ^GHU^6XEZHOOHQOlQJHQJLWWHU^OLH^ preferably between 10 nm and 500 nm, more preferably between 50 nm and 400 nm and particularly preferably between 100 nm and 350 nm. The security against counterfeiting of nanostructures is based in particular on the fact that their production requires high-resolution systems and processes, such as those used in electron beam lithography. Compared to embossed holograms, the nanostructures to be created are an order of magnitude smaller. In addition, the various molding steps for film production, which are primarily located in the process of embossing tool production, must be predictably true to form so that the structures embossed on the film can be seen by the viewer, for example, in the desired color.However, regular subwavelength gratings can still be produced using conventional lithography systems and even direct laser exposure – albeit under more difficult conditions. Counterfeit protection is therefore not yet optimal. In general, subwavelength gratings are more difficult to produce than hologram gratings with correspondingly larger grating periods, as they require a higher resolution. The smaller the structures, the higher the resolution of the systems used must be. For example, laser exposure systems must be more stable (with respect to mechanical vibrations, temperature) and more precise (with respect to positioning accuracy) and use lasers with a shorter wavelength. However, extended regular subwavelength gratings are in most cases easier to produce than arrays of gratings with varying parameters.The smaller the uniformly structured surface areas and the greater the number of different parameters, the less cost-effective or more difficult direct exposures become. This also becomes increasingly difficult for laser printers: even if they can still generate the required grating periods, these will generally deviate from the precise shapes originally designed. Even electron beam systems, which can in principle generate such structures, require more complex data processing. EP 2447743 A1 discloses a security element of the type mentioned above, which provides pixels that cannot be resolved with the naked eye and have subwavelength gratings coated with a high-refractive dielectric.To ensure that the optical impression of a pixel does not change when the security element is rotated perpendicular to its surface, each pixel is constructed from sub-pixels whose sub-wavelength gratings have an individual azimuthal alignment within the pixel. The document refers to the pixels thus obtained as "isotropic pixels." EP 2229287 A2 relates to a security feature whose surface areas filled with sub-wavelength gratings light up at specific tilt angles and lead to a geometric distortion of the perceivable shapes. Therefore, the corresponding surface areas are pre-distorted so that they appear undistorted in correct proportions at the specific tilt angles. Security features that contain regular nanostructures, i.e.Structures in the form of subwavelength gratings, which are used, sometimes have a very disturbing optical effect: At high tilt angles or when light is incident at a certain angle, the observer sees the subwavelength gratings light up brightly, as the first order of diffraction becomes visible. This can severely impair color perception because the original color of the surface area filled with this subwavelength grating is suddenly no longer recognizable. Instead, the first order of diffraction outshines the color originally assigned to this surface area. Another irritating factor is that, due to the sudden brightness of this particular sub-area, the colors of the neighboring surface areas are no longer clearly visible. Such nanostructures with a regular arrangement, i.e. subwavelength grating structures, can also be found, for example, in EUR banknotes.The security films of some denominations contain individual surface areas that illuminate green when tilted at a high tilt angle because they are filled with a 300 nm cross grating. The object of the invention is to provide a security feature with the generation of structural colors based on subwavelength gratings, upon observation of which no impairments caused by diffraction effects are perceived. The invention is defined in the independent claims. The dependent claims relate to preferred developments. The security element for valuables or valuable documents has a subwavelength grating structure on a substrate. This can be a one- or two-dimensional periodic subwavelength grating structure; it displays a color in a vertical plan view. A plurality of subregions are formed that are not resolvable with the naked eye.Each sub-area has a sub-wavelength grating structure that is homogeneous or uniform within the sub-area. Between the sub-areas, the sub-wavelength grating structures differ in at least one structural parameter that influences light diffraction. The multitude of sub-areas, which are not resolvable with the naked eye, cover a surface area of ​​the security element. The sub-areas provided on the surface area differ with respect to at least one structural parameter such that an observer in the surface area occupied by the sub-areas does not perceive any rotation angle-dependent diffraction effect when tilting the security element. This tilting occurs around a tilt axis that lies in a substrate plane, thus changing the elevation angle. This concept therefore provides that the sub-areas can certainly change in terms of color impression with the tilting.However, since the sub-regions differ from one another with regard to the structural parameters within the surface area, at large tilt angles, no significant diffraction-related change in the color impression occurs across the surface area as a whole, i.e. averaged over the sub-regions occupying the surface area, when the security element is rotated about an axis perpendicular to the substrate plane. A bright illumination of the first diffraction order, as occurs with an (extended) homogeneous grating when tilted about an axis parallel to the grating lines at high tilt angles, does not occur to a significant extent in the security element according to the invention. The color constancy is particularly high when the sub-wavelength gratings generate the color based on plasmon resonance, i.e. in particular have a metallically coated relief profile that provides the sub-wavelength grating structure.Aluminum, gold, silver, copper, chromium, nickel, and / or iron, as well as their alloys, can be used as the coating material. The thickness of the metallic coating is advantageously between 5 nm and 200 nm, preferably between 8 nm and 150 nm, particularly preferably between 15 nm and 80 nm. The metallic coating advantageously forms a largely opaque coating. Furthermore, it is also possible to achieve further effects based on a different functional principle, in particular interference effects, by combining it with additional thin layers. Optionally, an HRI layer, a multilayer coating (e.g., to complete a color-shift system with absorber / dielectric / reflector), or even a layer sequence of HRI and LRI layers (HRI = High Refractive Index; LRI = Low Refractive Index) can be applied to the metallic coating.The HRI layer is preferably made of ZnS or TiO2, and the LRI layer is preferably made of a polymer, SiO2, or MgF2, for example. The structural parameter individualizes the sub-regions within the surface area. Several parameters of the sub-wavelength grating structure are suitable for this purpose and can be used individually or in combination for individualization. These include the azimuthal orientation, the period, the outline of elevations and / or depressions of the sub-wavelength grating structure regularly arranged on a base area (e.g., rotationally symmetric outline shapes; non-rotationally symmetric, e.g., rectangular, in particular square outline shapes, ellipses, parallelograms, etc.). In particular, it is possible to combine several sub-regions into groups, whereby within a group a first parameter individualizes the sub-regions, and the groups differ from one another by a second parameter.Likewise, it is possible for the sub-regions in each group to be individualized by a different parameter. For example, in the sub-regions of one group, individualization can result from an azimuthal orientation, and in another group from a variation in the period. Particularly preferably, adjacent, different sub-regions of the surface area have a variation in the azimuthal orientation between 0° and 360°. Likewise particularly preferably, the period of adjacent, different sub-regions of the surface area differs by a value of a maximum of 10% (e.g., 30 nm at 300 nm), preferably of a maximum of 5% (e.g., 15 nm at 300 nm). In an advantageous embodiment, the structural parameter in the surface area is varied essentially isotropically. In particular, a random or pseudo-random variation of the parameter within a variation range is possible.Pseudorandom numbers are sequences of numbers that appear random but are calculated using a deterministic algorithm and are therefore not truly random numbers in the strict sense. Nevertheless, pseudorandom numbers are widely used because the statistical properties of a pseudorandom number distribution, such as the equal probability of individual numbers or the statistical independence of consecutive numbers, are generally sufficiently irregular for practical purposes, such as the pseudorandom variation of the azimuth angle, and pseudorandom numbers, unlike true random numbers, are easy to generate with computers. The security element can advantageously have multiple surface areas. The surface areas typically form individual regions within a motif, each of which has a uniform color according to the chosen design.If the motif is an apple tree, for example, a first surface area can form the brown trunk, a second surface area the green crown, and a third surface area a red apple. In a more detailed design, the crown itself could consist of different surface areas in varying shades of green, or the color of the apples could vary between red and yellow, so that each apple represents its own surface area with an assigned color from the spectrum between red and yellow. Each apple could also consist of different surface areas with colors from the spectrum between red and yellow. The design of the surface area by means of several sub-areas, which are individualized by the at least one structural parameter, can advantageously be combined with a micromirror arrangement, wherein the surface areas are formed on the micromirror arrangement.Particularly preferably, the surface regions are combined with a micromirror arrangement in such a way that the surface regions are each designed as micromirrors of this micromirror arrangement. The subwavelength grating structure of the subregions of each micromirror imparts a specific color that is largely independent of the elevation angle. The micromirror arrangement thus creates a bright or colored motif. Tilting the security element then results in a desired effect being created, whereby the color or color intensity of the individual micromirrors is not affected by diffraction effects of the color-imparting subwavelength structure due to the individualized subregions. In these cases, the surface regions are preferably not resolvable with the naked eye, since the individual micromirrors are preferably below the resolution limit.In other embodiments, larger surface areas are created that are resolvable with the naked eye, but nevertheless have a uniform color effect resulting from a subwavelength structure formed in a plurality of sub-areas that are not resolvable with the naked eye and are difficult for a counterfeiter to replicate. In a further particularly preferred embodiment, the surface areas correspond to micromirror pixels in which several micromirrors with a uniform orientation are located. The pixel size of the micromirror pixels is typically in the range of 10 μm to 30 μm, in particular 20 μm. The sub-areas can coincide with the micromirrors within a micromirror pixel. However, this is not absolutely necessary. The boundaries of the micromirrors and the sub-areas can also run independently of one another within a micromirror pixel.To avoid interfaces and the resulting weakening of the color effect, the sub-regions should not be significantly smaller than the micromirror pixel size. Typical dimensions for the sub-regions are therefore, for example, at least approximately 4 to 5 μm per sub-region with a micromirror pixel dimension of approximately 10 μm x 10 μm. In yet another particularly preferred embodiment, the surface regions each correspond to a plurality of micromirror pixels (i.e. regions in which micromirrors with a uniform orientation are located). The pixel size of the micromirror pixels is also typically in the range of 10 μm to 30 μm, in particular 20 μm. For example, each micromirror pixel has dimensions of 20 μm x 20 μm. Preferably, the subregions coincide with the micromirror pixels within a surface area. However, this is not mandatory.The boundaries of the micromirror pixels and the sub-areas can also run independently of one another within the surface area. In typical embodiments, several micromirror pixels have the same color. It is not absolutely necessary for all micromirror pixels to have the same orientation or inclination (or for the orientation / inclination of the micromirrors within the micromirror pixels to be the same for all micromirror pixels). Rather, the orientation / inclination of the micromirror pixels can vary within the surface area. The surface areas to be filled with structural colors using sub-wavelength gratings are divided into sub-areas whose size is below the resolution of the human eye. These sub-areas are filled with sub-wavelength gratings that are clearly defined by a specific set of parameters and fill their respective sub-areas homogeneously.The subwavelength gratings in the subregions can be either one- or two-dimensional periodic subwavelength gratings. The subwavelength gratings of adjacent subregions within a surface area differ from one another with respect to at least one structural parameter influencing the diffraction properties. This difference has little or no visual effect when viewed vertically, so that in this state, the generated structural color appears the same or at least very similar to the structural color generated by the individual subregions. In particular, when illuminated and viewed at shallow angles, no maxima attributable to the first diffraction order occur, which could significantly alter or outshine the generated structural colors. The absence of the bright illumination of the first diffraction order is detectable with the naked eye without any aids.Furthermore, any residual diffraction that may occur occurs azimuthally uniformly and with significantly reduced intensity. The disclosed security feature also represents a higher hurdle against counterfeiting attempts. The diffraction and / or resonance properties of the subwavelength gratings are essentially defined by the following structural parameters, which can be used individually or in combination to individualize the sub-regions: - Period between 10 nm and 500 nm, preferably between 50 nm and 400 nm, particularly preferably between 100 nm and 350 nm; - Depth between 50 nm and 400 nm, preferably between 80 nm and 300 nm; - One-dimensional (linear) or two-dimensional gratings; for two-dimensional gratings, for example,Rectangular, square, hexagonal, or parallelogram-shaped grid arrangement; - azimuthal alignment of the grids in the plane of the security feature, relative to a predefined direction; - Profile shape of the relief structure: sinusoidal, rectangular (binary) structures, or other profile shapes with concave and / or convex sections; a periodic arrangement of nanoholes or nanodots with various outline shapes is also possible. The grids are preferably uniformly coated with metal, as this creates a clearly recognizable color that does not change significantly over a larger angular range. In another preferred embodiment, metallization is carried out in regions. For example, the top surfaces of embossed elevations ("columns") are provided with a coating, while the remaining surface areas have no coating. The reverse embodiment—regularly arranged holes in a metal layer—is also possible.The azimuth angle, i.e., the orientation of the regular subwavelength gratings in the plane defined by the substrate, is particularly well-suited as a structural parameter to be varied in the subregions, since, under non-polarized illumination, it has largely no influence on the color generated by plasmon resonance effects when viewed perpendicularly and essentially only affects diffraction. Particularly preferably, the azimuth angle of the subregions filled with the subwavelength gratings, which together represent the entire surface area to be filled with an essentially uniform structural color, varies randomly or pseudo-randomly within a predefined range of variation.This means that the azimuth angle of each sub-area can assume any value between 0 and 360°, so that due to the large number of sub-areas into which an essentially homogeneously colored surface is divided, sub-wavelength gratings with diverse azimuth angles and approximately the same azimuthal distribution, i.e., essentially isotropic, occur across the entire angular range. This ensures that an observer cannot see any change due to diffraction effects, or at least no dependence of the diffraction color on the azimuthal direction in which the security feature lies or from which the security feature is viewed. Furthermore, other structural parameters can also be varied across the sub-areas. It is also possible to vary the sub-areas occurring in differently colored surface areas according to different structural parameters, e.g.to vary the period in a surface area filled with a blue structural color and to vary the azimuth angle from sub-area to sub-area in a surface area filled with a yellow structural color. Furthermore, it is also possible to vary several grating parameters simultaneously within a surface area, whereby in this case too the structural parameters azimuth angle and period are preferred. The sub-areas have - as mentioned above - a lateral extent below the resolution of the human eye. Their size is therefore below 300 μm, preferably below 100 μm, particularly preferably below 50 μm. This applies to their maximum extent in at least one spatial direction, i.e. the sub-areas can be very narrow, elongated lines, although sub-areas whose dimensions are below the resolution limit in all directions are preferred.The subregions can have irregular outlines, but they can also appear in the form of regularly arranged pixels. In special embodiments, the subject matter of the invention can also be combined with one or more additional subregions in which the grid parameters are not varied at all. For example, it can be provided that small subregions (e.g., subpixels with 10 μm edge length) are present over a large area in which the azimuth angles are randomly selected, while in other subregions, a grid with the same azimuth angle is present (e.g., as a value number readable with the naked eye).In a vertical view, the small sub-regions with varying azimuth angles and the other sub-regions with identical azimuth angles, which are also preferably not resolvable with the naked eye, are indistinguishable or at least barely distinguishable. However, when viewed at a suitable angle (when tilting about an axis parallel to the grating lines), a further representation becomes visible here due to the otherwise undesirable first order of diffraction of the macroscopic sub-region formed from the other sub-regions with identical azimuth angles. Such a combination also makes it easier to unambiguously verify the feature according to the invention: The absence of brightly illuminating first-order diffraction effects may not always be unequivocally verifiable, since in practice this may be due, for example, to poor lighting conditions or to the fact that an observer is not looking closely enough.However, if a grating with fixed grating parameters is provided in a macroscopic sub-area formed from further sub-areas, i.e., sub-areas visible to the naked eye, an observer will obtain a clear result if these further sub-areas illuminate brightly in the first diffraction order, but the other sub-areas show no such effect. According to a further example, it can be provided that in an outline of a motif (for example in the form of a symbol readable with the naked eye), which in one dimension has lateral dimensions below the resolution limit of the eye (e.g., 60 μm), there are small sub-areas (e.g., subpixels with 10 μm edge length) in which the azimuth angles are randomly selected, while in another sub-area there is a (continuous) sub-wavelength grating.As in the aforementioned example, the small sub-areas with varying azimuth angles and the wider sub-area are indistinguishable from a vertical view. However, when viewed at a shallow angle, the residual diffraction occurring to a small extent in the small sub-areas with varying azimuth angles reveals a motif representation. However, when viewed from a direction perpendicular to the grating lines of the wider sub-area (or when tilted about an axis parallel to the grating lines), this motif is outshone by the first order of diffraction, so that the motif formed by the outline disappears again in this viewing situation, which can be achieved by changing the angle of rotation. Such a combination also facilitates the verification of the feature according to the invention.By providing the sub-area containing the small sub-areas with varying azimuth angles with lateral dimensions in one dimension that are below the resolution limit of the eye, it is also ensured that the surface area as a whole can be perceived with a uniform color or brightness. A subdivision of the (macroscopic) further sub-area into sub-areas that are not perceptible to the naked eye is therefore not absolutely necessary to achieve an overall uniform appearance. Accordingly, a continuous sub-wavelength grating can be provided throughout the entire further sub-area. The production of sub-wavelength gratings is carried out, for example, by the following process: - Electron beam lithography; - Production of an embossing tool through galvanic molding steps; - Embossing in thermoplastic or radiation-curing, in particular UV-curing, embossing lacquer; - Metallizing (e.g.with Al, Au, Ag, Cr, Ni, Cu, Fe or their alloys), possibly additionally with an HRI layer (e.g. ZnS); a multi-layer coating is also conceivable and can be used advantageously (e.g. colorshift system with absorber / dielectric / reflector, where the subwavelength structure changes the colorshift color); - optional embedding of the coated embossed structures (e.g. with protective varnish).In addition to the visually perceptible difference between the arrangements of subwavelength gratings described here compared to the state of the art, in which the gratings homogeneously fill uniformly colored surface areas with a constant set of grating parameters, the security features according to the invention have another advantage: they are practically impossible to replicate using direct laser exposure utilizing interference at the sample location, because the large number of variably filled sub-areas and the small size of these sub-areas would make direct exposure too difficult and costly. Furthermore, the small grating periods also rule out other optical methods for origination. Production using electron beam systems is possible, but requires greater effort in data processing. This represents a double hurdle for counterfeiters.Electron beam systems are inherently very expensive and therefore difficult for counterfeiters to access. Furthermore, the additional software required to process the data presents a further obstacle. These nanostructures are particularly interesting in security features where they are combined with microstructures. While the microstructures primarily ensure high dynamics and the associated optical variability, the subwavelength grating structures impart characteristic colors to the feature. The measures described so far for generating colored representations for security features based on structural colors are therefore particularly effective when superimposed with microstructures. The structures are more complex and difficult to manufacture than the subwavelength gratings known from the state of the art.The microstructures make an additional contribution to suppressing diffraction phenomena, since the diffraction condition—that is, when an incident light beam is diffracted toward the observer—also depends on the orientation of the microsurface on which a subwavelength grating is placed. Furthermore, the microstructures ensure the dynamics, while the subwavelength gratings provide the corresponding colors. In one series of experiments, for example, two motifs of a binary micromirror tilt image with micromirrors noisy in one direction were additionally equipped with nanostructures to impart color. The nanostructures on the micromirrors consisted of subwavelength cross gratings with a sinusoidal profile, which were divided into regular, 20 μm subpixels as sub-areas. The azimuthal orientation of the subwavelength cross gratings changed randomly from subpixel to subpixel.While the remaining (azimuthally evenly distributed) diffraction was still faintly visible in the tilted image with the least noisy micromirrors, this diffraction decreased with increasing variation in the mirror alignment and was ultimately no longer detectable at the most noisy level. In a particularly preferred embodiment of the manufacturing method, the azimuth angle is varied in the sub-regions, i.e., an alignment of the regular periodic sub-wavelength grating structure in the plane defined by the substrate is changed from sub-region to sub-region. The individual sub-regions particularly preferably contain two-dimensionally periodically arranged sub-wavelength grating structures, which are preferably applied to the substrate in a rectangular, square, hexagonal, or parallelogram-shaped grid arrangement.The two-dimensional periodic subwavelength grating structures have regularly arranged elevations and depressions, wherein the elevations are in particular not rotationally symmetrical in the top view of the security element, e.g., rectangular, particularly preferably square. The elevations are, of course, not limited to square or rectangular shapes in their top views. Other, non-rotationally symmetrical shapes are also possible, such as ellipses or parallelograms. For ease of manufacture, it is preferred that, although the azimuth angle changes from sub-area to sub-area, the alignment of the elevations in the top view is always the same in all sub-areas, regardless of the azimuth angle. This is explained in more detail below using an example: In one example, the elevations are rectangular in the top view. The edges of all elevations are parallel to one another, i.e.The rectangles visible in plan view all have the same orientation. To vary the azimuth angle, the grid arrangement is rotated by a specific angle from sub-area to sub-area, whereby the parallelism of the edges of the elevations is maintained across all sub-areas. Each elevation with a specified register point, which is the same for all elevations, is fixed to an intersection point of grid lines of a grid, so that the register point lies exactly on this intersection point. The register point can, for example, be the center of an elevation that is rectangular in plan view, but any other point can also be used as a register point. The sub-areas differ in the orientation of the grid arrangement, but not in the orientation of the elevations, which each lie with their register point on the respective intersection point.The described preferred manufacturing method is also suitable for one-dimensional periodic subwavelength grating structures. The horizontal or vertical spacing of the elevations is then selected to be zero, resulting in a quasi-one-dimensional periodic subwavelength grating structure with grating ridges and grating gaps. With the preferred manufacturing method, the subwavelength grating structure can be exposed more quickly and thus also manufactured more easily due to the identical alignment of the non-rotationally symmetric elevations in all subregions. The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as limiting.For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the embodiments may also be applicable to other embodiments. To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. The figures show: Fig. 1 a schematic representation of a banknote with a security element, Fig.2 is a perspective schematic representation of the security element to illustrate a tilting effect, Fig. 3 is a plan view of the security element of Fig. 1, Fig. 4 is a sectional view through a partial area of ​​the security element of Fig. 3, Fig. 5 is a further embodiment of a banknote with two security elements, similar to that of Fig. 4, Fig. 6 is a further plan view of a security element, Figs. 7 to 9 are plan views of a partial area of ​​the security element, Fig. 10 is a detailed view of an elevation in plan view, and Fig. 11 is a plan view of a partial area of ​​a security element. Fig. 1 schematically shows a banknote 2 or another value document which is provided with a security element 4 for copy protection. The security element 4 presents a motif to a viewer and for this purpose has a plurality of pixels or surface areas 6.The security element 4 can be formed directly on banknote paper during production. Alternatively, it is possible to provide it on a separate substrate that is applied to the banknote 2, for example, by previously forming the security element 4 as a so-called transfer element. Such a procedure is known to those skilled in the art. Fig. 2 shows a plan view of the security element 4, with only one of the pixels 6 being shown for the purpose of explanation. The security element 4 and thus the pixel 6 is viewed, for example, largely perpendicularly from a view direction 8. All pixels 6 of the security element 4 have a subwavelength grating that will be explained later, with the previously explained definitions of terms naturally also applying to the descriptions of the exemplary embodiments in Figs. 1-11.If the security element 4 is tilted about an axis 10, which lies in a plane defined by the planar security element 4 and / or the planar banknote 2, the viewing direction 8 changes accordingly. Nevertheless, the color presented by the pixel 6 remains essentially unchanged over a wide viewing angle range, the boundaries of which are schematically designated 12 and 14. Subwavelength gratings are therefore also optically variable, although their optical variability is less pronounced than is the case with conventional diffraction gratings. In particular, no diffraction effects are perceptible over a wide tilt angle range, and furthermore, no azimuth-dependent diffraction phenomena occur. This property of each pixel 6 is achieved in that, as shown in Fig.3 schematically shows one of the pixels 6 in plan view, formed from a plurality of partial regions 16, here in the form of subpixels, each of which is formed by a subwavelength grating structure. The individual partial regions 16 differ with regard to a parameter influencing light diffraction. In the plan view of Fig. 3, this is the azimuthal orientation or a longitudinal direction 18 in which the one-dimensionally periodic subwavelength grating structure extends, as an example here. In an embodiment not shown here, the grating lines of a one-dimensionally periodic subwavelength grating structure can merge from one partial region continuously in the form of curved lines into the grating lines of another partial region or other partial regions with the same period but rotated azimuth.In this way, discontinuities at the boundaries of the partial areas covered with different sub-wavelength gratings can be avoided or at least minimized. Fig. 4 shows a sectional view through the partial area 17 (Fig. 3) along a horizontal axis. The sectional view in Fig. 4 shows that the security element 4 is formed on a substrate 20 on which an embossing lacquer layer 22 is arranged, into which a relief of a sub-wavelength grating structure 24 is embossed. This is periodic at least in the sectional plane and extends, for example, in the longitudinal direction perpendicular to the plane of the drawing. It consists of a plurality of elevations 28 and depressions 30, which in the example shown follow one another periodically in one dimension. A two-dimensional, periodic grating structure is of course equally possible. In this case, there are generally two directions, each of which can serve as the longitudinal direction 18.The relief structure is provided with a metallization layer 26. The partial regions 16 of the pixel 6 can be understood as subpixels. In this case, all partial regions 16 of a pixel 6 produce the same color, and are therefore designed with regard to their subwavelength grating 24 to present the same or at least essentially the same color. However, they differ with regard to the longitudinal direction 18 along which the subwavelength grating structure 24 extends. In the illustrated embodiment in Fig. 3, a subdivision into 36 partial regions 16 is provided for the pixel 6, the longitudinal direction 18 of which differs from one another by 10 degrees. The longitudinal directions 18 of the subwavelength grating structures 24 are thus evenly distributed between 0° and 360° in the exemplary embodiment. The partial regions 16 which follow one another with regard to the variation in the longitudinal direction 18 do not have to be regularly arranged, as is the case in Fig.3 column-wise from top left to bottom right. The individual longitudinal directions 18 can also be distributed completely randomly or pseudo-randomly among the individual sub-regions 16. Furthermore, the sub-wavelength grating structure 24 is neither limited to a one-dimensional periodic structure, nor is the variation limited to a variation of the longitudinal direction 18. Other sub-wavelength grating structures can equally well be used and other parameters of this sub-wavelength grating structure can be varied, as explained in the general part of the description. In particular, it is possible to realize the sub-wavelength grating structure 24 by a two-dimensional periodic arrangement of elevations and / or depressions and / or to vary a period between the individual sub-regions 16 as a parameter, rather than a longitudinal direction 18. Fig.Fig. 5 shows that the division into subregions 16 is not directed at the subdivision of pixels 6, i.e., surface areas that cannot be resolved with the naked eye, but can equally be used for larger areas 32, 34, 35 that are divided into subregions 16 that cannot be resolved with the naked eye. Fig. 5 shows by way of example that the subregions 16 do not necessarily have to be regularly arranged or have the same basic structure. Fig. 6 shows a further embodiment of the security element 4, which in this variant is designed as a micromirror arrangement 36 constructed from individually oriented micromirrors 38. Each of the micromirrors 38 comprises a plurality of subregions 16 that can differ from one another, for example in the longitudinal direction 18 of the subwavelength grating structure 24, as explained with reference to pixel 6 in Fig. 3.Of course, the options for how the individual sub-regions 16 can differ from one another, which were explained with reference to Figs. 3 and 4 and are also mentioned in the general part of the description, also apply equally to the sub-regions 16 on a micromirror 38. The design of the micromirrors 38 by a plurality of sub-regions 16, which differ in at least one parameter, has the advantage that the individual micromirrors 38 of the micromirror arrangement 36 show the desired color regardless of their orientation and the effect does not occur that with a certain orientation of the micromirror 38, which ultimately corresponds to a tilt angle according to Fig. 2, the desired color is not visible at all and / or is impaired or disturbed by a higher diffraction order, for example the first diffraction order. Fig.Figure 11 shows a plan view of a section of another embodiment of a security element which, like the security element shown in Figure 6, is designed as a micromirror arrangement 36 composed of individually oriented micromirrors 38. In contrast to the embodiment shown in Figure 6, the micromirrors are present in micromirror pixels, each of which contains several micromirrors with a uniform azimuthal orientation (and inclination) (indicated in the figure by corresponding hatching). Several micromirror pixels have the same color, specified by a subwavelength grating structure.The division into sub-regions 16 that cannot be resolved with the naked eye occurs here for larger regions 32 that can be perceived with the naked eye, one of which is shown in detail with a dashed outline included in the figure solely for illustration purposes (and with the longitudinal direction 18 of the sub-wavelength grating structures). The sub-regions 16 of the regions 32 are designed with regard to their sub-wavelength grating so that they present the same color. However, as explained using pixel 6 in Fig. 3, they differ from one another with regard to a parameter that influences light diffraction. In the plan view of Fig. 11, this is the azimuthal orientation or a longitudinal direction 18 in which the one-dimensional periodic sub-wavelength grating structure extends, as an example here.In the exemplary embodiment, the micromirror pixels each coincide congruently with the sub-regions 16 of the regions 32. The micromirror pixels of the region 32 all have the same orientation and inclination, i.e. the orientation of the micromirrors within the micromirror pixels is the same for all micromirror pixels of the surface region. However, this is not absolutely necessary. According to an embodiment not shown here, the orientation and / or inclination of the micromirror pixels in the surface region can also vary. Figs. 7 to 9 show plan views of sub-regions 16a to 16c of the security element 4, and Fig. 10 shows a raised portion 28 in detail. Figs. 7 to 10 serve to illustrate a preferred manufacturing method for the security element 4, which is explained below. Fig. 7 shows a plan view of a first sub-region 16a of a pixel 6.The partial area has a two-dimensional subwavelength structure in the form of depressions 30 and elevations 28, the latter being arranged in a square grid arrangement. This grid arrangement forms a first grid 40. The elevations 28 are arranged in the first grid 40, for example, such that a center point M of each elevation 28 lies at an intersection point of horizontal and vertical grid lines of the first grid 40. The center point M serves to illustrate a registration of the elevations 28 in the first grid 40. For the sake of simplicity, the center point M is only drawn for one elevation 28. The first grid 40 likewise serves only to illustrate the arrangement of the elevations 28 in the partial area 16a; only in this way is it recognizable in the plan view of the security element 4. The grid arrangement according to the first grid 40 is, by way of example, square, i.e.The horizontal and vertical grid lines are at a 90° angle to each other. However, grid arrangements in a hexagonal grid, for example, are also possible (not shown). Fig. 8 shows a second partial area 16b of the pixel 6 in plan view, in which the azimuth angle is changed compared to Fig. 7 based on a predetermined value. This is achieved in that the elevations are arranged in a second grid 42 which is rotated by a first angle 44 relative to the first grid 40, but otherwise has the same structure. In the exemplary embodiment in Fig. 8, the angle is 4410°. This rotation of the second grid 42 relative to the first grid 40 by the angle 44 is shown as a detail at the bottom right of Fig. 8 for illustrative purposes. Regardless of the rotation of the second grid 42 relative to the first grid 40 by the angle 44, the orientation of the elevations 28 in Figs. 7 and 8 remains the same, ie in Fig.8, the edges of the upper side of the elevations 28 are parallel to those of the previous partial area 16a; they do not rotate with the second grid 42. Fig. 9 shows a third partial area 16c of the pixel 6 in plan view. As in Figs. 7 and 8, elevations 28 are registered with their centers M on the intersection points of the grid lines of a third grid 46. This corresponds in its basic structure to grids 40 and 42, but is rotated by a second angle 48 of 30° with respect to grid 40. Regardless of the rotation of the third grid 46, the orientation of the elevations 28 remains the same in Fig. 9, i.e., in Fig. 9 too, the edges of the upper side of the elevations 28 are parallel to those of the partial areas 16a and 16b. The rotation of the third grid 46 relative to the first grid 40 by the second angle 48 is shown as a detail at the bottom right edge of Fig. 9. Fig. 10 shows a single elevation 28 in plan view.The elevation 28 has the center point M. Additionally, the first grid 40, the second grid 42, and the third grid 46 are shown, which are registered with the center point M of the elevation 28 such that the center point M is located at the intersection point of the grid lines of the grids 40, 42, 46. Also shown are the first angle 44 and the second angle 48, by which the second grid 42 is rotated around the first grid 40 and the third grid 46 is rotated around the first grid 40, respectively. Figs. 7 to 9 show two-dimensional periodic grating structures with elevations 28 and depressions 30, arranged, for example, in a rectangular grid arrangement in the form of a square grid 40, 42, 46. It is possible to set the horizontal or vertical distances between the elevations 28 equal to zero and thus create a one-dimensional periodic grating with grating bars and gaps (this embodiment is not shown).7 to 9 each show a sub-region 16a, 16b, 16c with a different azimuth angle. However, the edges of the elevations or the edges of the upper sides of the elevations remain the same in each of Figs. 7 to 9 (i.e., in all sub-regions), namely, in the embodiments shown, for example, parallel to the grid lines of the first grid 40. Fig. 10 summarizes that, regardless of the grid 40, 42, 46, the orientation of the elevation 28 always remains the same in the preferred manufacturing process. To adjust the azimuth angle, the orientation of the elevations is retained, but the second grid 42 is rotated by the azimuth angle relative to the first grid 40 or the third grid 46 is rotated by the azimuth angle relative to the first grid 40.In this manufacturing process, the orientation of the regular periodic subwavelength gratings as a whole is changed to produce different azimuth angles in different sub-areas 16, but the orientation of the elevations 28 remains the same.

[0002] List of reference symbols Banknote 4 Security element Pixel 8 View direction 10 Axis 12, 14 Boundary 16, 17 Partial area 18 Longitudinal direction 20 Substrate 22 Embossing lacquer layer 24 Subwavelength grating structure 26 Metallization 28 Elevation 30 Depression 32, 34, 35 Area 36 Micromirror arrangement 38 Micromirror 40 First grating 42 Second grating 44 First angle 46 Third grating 48 Second angle

Claims

Patent claims 1. Security element for valuables or valuable documents (2), which has a subwavelength grating structure (24) on a substrate (20) which, in a vertical plan view (8), displays a color generated on the basis of plasmon resonance, wherein a plurality of partial regions (16, 17) which cannot be resolved with the naked eye are formed, in each of which the subwavelength grating structure (24) is formed uniformly, wherein the subwavelength grating structures (24) differ between the partial regions (16, 17) in at least one structural parameter influencing light diffraction, and the plurality of partial regions which cannot be resolved with the naked eye cover a surface region (6, 32) of the security element (4), characterized in that the partial regions (16, 17) in the surface region (6, 32) are in which distinguishes at least one structural parameter in such a way that an observer in the sub-areas (16,17) does not perceive any rotation angle-dependent diffraction effect when tilting the security element (4) about at least one tilt axis (10) lying in a substrate plane.

2. Security element according to claim 1, characterized in that the subwavelength grating structures (24) have a metallically coated relief profile (28, 30), wherein the metallic coating (26) consists in particular of aluminum, gold, silver, copper, chromium, nickel and / or iron or an alloy of these metals.

3. Security element according to claim 1 or 2, characterized in that the structural parameter in which the subwavelength grating structures (24) of the partial regions (16, 17) differ comprises at least one of the following parameters of the subwavelength grating structure (24): azimuthal, - 2 - Alignment, period, outline of elevations and / or depressions of the subwavelength grating structure regularly arranged on a base area.

4. Security element according to one of the above claims, characterized in that the subwavelength grating structures (24) of the partial areas (16, 17) differ in their azimuthal alignment, with adjacent, different partial areas of the surface area (6, 32) having a variation in the azimuthal alignment between 0° and 360°.

5. Security element according to one of the above claims, characterized in that the subwavelength grating structure (24) is a two-dimensional periodic subwavelength grating structure, with the subwavelength grating structure (24) in particular having a rectangular, square, hexagonal, or parallelogram-shaped grid arrangement (40, 42, 44).A security element according to claim 5, characterized in that the subwavelength grating structure has regularly arranged elevations and / or depressions, wherein the subwavelength grating structures (24) of the partial regions (16, 17) differ in the orientation of the grid arrangement (40, 42, 44).

7. A security element according to one of the above claims, characterized in that the structural parameter is varied essentially isotropically in the surface region (6, 32).

8. A security element according to one of the above claims, characterized in that it has a plurality of surface regions (6). - 3 - 9. Security element according to one of the above claims, characterized in that the surface region(s) (6) are formed on a micromirror arrangement (36).

10. Security element according to claim 8, characterized in that the surface regions are each formed as a micromirror (38) of a micromirror arrangement (36), as a micromirror pixel having a plurality of micromirrors (38) of a micromirror arrangement (36) with a uniform orientation, or as a plurality of micromirror pixels, each having a plurality of micromirrors (38) of a micromirror arrangement (36) with a uniform orientation, wherein the subwavelength grating structure (24) of the partial regions (16, 17) of each micromirror (38) or each micromirror pixel or each plurality of micromirror pixels imparts a specific color to the micromirror arrangement (36) such that the micromirror arrangement (36) creates a colorful or colored motif.Security element according to one of claims 1 to 10, characterized in that the surface area(s) (6, 32) is / are resolvable with the naked eye.

12. Security element according to one of the above claims, characterized in that the plurality of partial areas (16, 17) differing in the at least one structural parameter is combined with one or more further partial areas whose structural parameters do not differ.

13. Method for producing a security element for valuable objects or valuable documents (2), wherein a subwave is formed on a substrate (20). - 4 - wavelength grating structure (24) is formed which displays a color generated on the basis of plasmon resonance, wherein a plurality of partial regions (16, 17) which cannot be resolved with the naked eye are formed, in each of which the subwavelength grating structure (24) is formed uniformly, wherein the subwavelength grating structures (24) between the partial regions (16, 17) differ in at least one structural parameter influencing the light diffraction and the plurality of partial regions (16, 17) which cannot be resolved with the naked eye cover a surface region (6, 32) of the security element (4), characterized in that in the surface region (6, 32) the partial regions (16, 17) differ in the at least one structural parameter such that an observer in the surface region (6, 32) when tilting the safety element (4) by at least one,in a substrate plane, no rotation angle-dependent diffraction effect is perceived.

14. Method according to claim 13, characterized in that the subwavelength grating structures (24) are provided with a metallically coated relief profile (28, 30).

15. Method according to claim 13 or 14, characterized in that the structural parameter in which the subwavelength grating structures (24) of the partial regions (16, 17) differ comprises at least one of the following parameters of the subwavelength grating structure (24): azimuthal orientation, period, outline of elevations and / or depressions of the subwavelength grating structure regularly arranged on a base area.

16. Method according to one of the above method claims, characterized in that the subwavelength grating structures (24) of the partial regions, - 5 - (16, 17) differ in their azimuthal orientation, wherein adjacent, different sub-regions (16, 17) of the surface region (6, 32) have a variation in the azimuthal orientation between 0° and 360°.

17. Method according to one of the above method claims, characterized in that the structural parameter in the surface region (6, 32) is varied essentially isotropically. 18.Method according to one of the above method claims, characterized in that a plurality of surface regions (6, 32) are provided, which are each formed in particular as a micromirror (38) of a micromirror arrangement (36), as a micromirror pixel having a plurality of micromirrors (38) of a micromirror arrangement (36) with a uniform orientation, or as a plurality of micromirror pixels, each having a plurality of micromirrors (38) of a micromirror arrangement (36) with a uniform orientation, wherein the subwavelength grating structure (24) of the partial regions (16, 17) of each micromirror (38) or each micromirror pixel or each plurality of micromirror pixels imparts a specific color to the latter, so that the micromirror arrangement (36) creates a colorful or colored motif. 19.Method according to one of claims 13 to 18, characterized in that the surface region(s) (6, 32, 34) is / are resolvable with the naked eye.

20. Method according to one of claims 13 to 19, characterized in that the subwavelength grating structure (24) is formed as a two-dimensional periodic subwavelength grating structure, wherein the subwave-. - 6 - wavelength grating structure (24), in particular in a rectangular, square, hexagonal, or parallelogram-shaped grid arrangement (40, 42, 44), is applied to the substrate.

21. The method according to claim 20, characterized in that the subwavelength grating structure has regularly arranged elevations and / or depressions, wherein the subwavelength grating structures (24) of the partial regions (16, 17) differ in the orientation of the grid arrangement (40, 42, 44).

22. An article or valuable object or valuable document (2) comprising a security element (4) according to one of claims 1 to 12.

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