Security element with colour-producing nanostructures and production method therefor

Abstract

The invention relates to a security element having a structured layer (8), which has a base area (20) and depressions (12) recessed with respect to the base area or elevations raised with respect to the base area, and a reflector layer (14) arranged on the structured layer (8), wherein the depressions (12) or elevations are designed, in terms of their extents along the base area (20), their vertical extent (t) perpendicular to the base area (20) and their arrangement on the base area (20), as colour-producing nanostructures (10), wherein the structured layer (8) has regions (16, 18; 25a-d) in each of which the vertical extent (t) of the depressions (12) or elevations along a direction (22) varies according to a non-constant function, with the result that a mixed colour is visible in plan view in each of the regions (16, 18; 25a-d).

Description

[0001] Security element with color-generating nanostructures and manufacturing processes for

[0002] The invention relates to a security element with color-generating nanostructures, comprising: a structured layer having a base area and recesses or raised elevations relative to the base area, and a reflector layer arranged on the structured layer, wherein the elevations or depressions are designed as color-generating nanostructures with regard to their extents along the base area, their vertical extent perpendicular to the base area and their arrangement on the base area, and wherein the structured layer has a plurality of regions that provide a colored motif or image, wherein the regions preferably form pixels.

[0003] The invention further relates to a method for producing a security element, the method comprising: producing a structured layer which has a base area and recesses or raised elevations relative to the base area, and arranging a reflector layer on the structured layer, the elevations or depressions being formed as color-generating nanostructures with regard to their extents along the base area, their vertical extent perpendicular to the base area and their arrangement on the base area, and the structured layer being formed with a plurality of regions which provide a colored motif or image, the regions preferably forming pixels.

[0004] Such a security element and such a method for its production are known from DE 102012105571 A1. Nanostructures with dimensions in the subwavelength range are known in the prior art, allowing the realization of colored images. For this purpose, elevations or depressions are arranged on a base surface. If this structure is mirrored, the color depends on the structural parameters of the elevations / depressions when they are imaged as nanostructures, i.e., structures with dimensions below the wavelength of visible light. Such structures are also known, for example, from DE 2009056934 A1, DE 102010048262 A1, DE 102010049832 A1, US 2012 / 236415 A1, WO 2019 / 140572 A1, or WO 2019 / 180460 A1. EP 3572852 A1 also discloses a security element with elevations / depressions that realize a color-generating nanostructure.

[0005] It is known to provide the color-generating nanostructures in areas that form pixels or subpixels. In each pixel or subpixel, a predetermined configuration of the elevations / depressions is provided with regard to extensions along the base area, vertical extension perpendicular to the base area, and arrangement on the base area - depending on the color that the pixel or subpixel is intended to generate. Such nanostructures generate a color that is generally influenced by the vertical extension. In the prior art, it is known to use constant vertical extensions in the pixels or subpixels, i.e., a constant height in the case of elevations or a constant depth in the case of depressions. In the case of subpixels, it is provided, for example, in the aforementioned DE 102012105571 A1 (Figs. 14b and 14c therein), to nest the subpixels, which differ in terms of the heights / depths of the elevations / depressions, in order to achieve a color difference by combining several, e.g.RGB subpixels are used to set a color for a pixel. The creation of regions with individual, vertical extensions that vary from region to region allows for the creation of a colored image. For this purpose, the regions are formed as subpixels. The subpixels each generate a color (e.g., red, green, or blue) and together set the desired color of the pixel.

[0006] Based on this, the invention is based on the object of specifying a security element based on color-generating nanostructures which has an improved color effect and, in particular, does not require sub-pixels.

[0007] The invention is defined in the independent claims. The dependent claims relate to preferred developments.

[0008] The security element has a structured layer having a base area and, relative to this, sunken, concave depressions or raised, convex elevations. Any reference to depressions below is to be understood merely as an example. A reflector layer is arranged on the structured layer. The elevations or depressions are formed as color-generating nanostructures with regard to their structural parameters, in particular their extents along the base area, their vertical extent perpendicular to the base area, and their arrangement on the base area, as is known in the prior art and in particular from the aforementioned EP 3572852 A1. The vertical extent is the height of the elevations and the depth of the depressions. The elevations or depressions may have cover surfaces that are parallel to the base area or tangent to the base area.The term "nanostructures" refers to the vertical dimensions of the elevations / depressions, which are usually less than 500 nm. The dimensions along the base surface and the distances between adjacent elevations / depressions, however, are usually in the micrometer range.

[0009] The nanostructure creates a colored motif or image and has multiple regions, where the vertical extension of the elevations or depressions varies along a direction according to a non-constant function in each region. This creates a color in each of these regions when viewed from above, which results in a mixed color due to the variation in the vertical extension. This term expresses that the color is created by mixing the effects of the varied, i.e., differently sized, vertical extensions of the elevations or depressions within the region. The term "mixed color" must be distinguished from color mixing, as achieved by multiple subpixels or pixels, because in these regions, the nanostructure creates exactly one individual color in each region, i.e., subpixels or pixels, due to the non-varied extension of the elevations or depressions without a mixing effect. Only when multiple pixels / subpixels are combined do they mix their individual colors.By varying the vertical extent of the elevations or depressions within an area (e.g. pixels), the color of each area is created by a mixed effect and not as a single color.

[0010] This simplifies the creation of an image because no structuring in sub-pixels is necessary for color generation; instead, each area, for example each pixel, can provide a mixed color. This color is set in the respective area by varying the vertical extent, in particular the area over which the variation extends and the distribution of the vertical extents in this area as well as the mean vertical extent, i.e. the mean value of the distribution. The colors contributing to the mix are set, among other things, by the degree of variation in the vertical extent. If the vertical extent varies over a large area, more colors are combined to form the mixed color than if the vertical extent varies over a smaller area.The mean depth of the varied vertical extension represents a color that, in terms of the hue-like wavelength, also lies in the middle of the hue-like wavelength among the mixed colors combined to form the mixed color. It is therefore preferred that, for each region, the center of the hue-like wavelength of the (mixed) color produced by the region be set by the mean depth of the varied vertical extension.

[0011] To produce a colored image, it is particularly preferred that the regions, or at least some of these regions, differ in terms of function. The difference can lie in a variation range of the vertical extensions. However, the difference can also lie in how the different vertical extensions are distributed in the varied region, for example whether large vertical extensions occur more frequently than small vertical extensions. This then places the center of gravity of the mixed color, with respect to the covered variation range of hue-like wavelengths, outside the center of the covered variation range of the hue-like wavelengths. It is therefore preferred to combine different regions that differ in terms of the variation range of the vertical extensions and / or the distribution of different vertical extensions in the varied region.

[0012] Optionally, especially in the latter case, the distribution of the vertical extensions is not symmetrical to the center of the covered variation range of the vertical extensions. Particularly good color mixing is achieved when the vertical extension of the elevations or depressions increases or decreases along the direction according to a gradient. In this way, a range of chromatic wavelengths is summarized, of which all chromatic wavelengths contribute equally to the mixed color if the gradient is a linear gradient. Individual chromatic wavelengths can contribute differently to the mixed color if the gradient is non-linear, for example, quadratic, etc.

[0013] A particularly preferred embodiment results when these nanostructures are combined with microstructures, i.e. with microstructure elements which have at least one direction extension above the wavelengths of the visible spectral range, e.g. of at least 5 pm, preferably 10 pm, 50 pm or more. It is then possible to form the regions as microstructure elements each equipped with a height profile (i.e. variation in height), e.g. inclined mirror surfaces. At least some regions differ from one another with regard to the height profile and / or the shape of the microstructure elements (in plan view of the base plane). The nanostructures are formed on the microstructure elements, the vertical extension of which varies across the respective microstructure element. In this embodiment, one region then corresponds to one microstructure element.

[0014] It is particularly preferred that the vertical extension increases or decreases with the height of the microstructure element, e.g., the mirror surface, since this allows for particularly simple production, which exploits local sensitivity differences of a photoresist that arise due to the microstructuring. This results in either elevations / depressions with a greater vertical extension at the upper edge of the mirror and elevations / depressions with a smaller vertical extension at the lower edge of the mirror, or vice versa. The decrease in the vertical extension runs parallel to the mirror inclination between the upper and lower edges of the mirror.

[0015] The microstructure elements, e.g. the mirror surfaces, can be designed in such a way that they produce a bulging effect, which is enhanced and colored by the color-imparting nanostructures, ie produces an improved 3D effect.

[0016] The direction along which the nanostructures vary in vertical extent is preferably aligned at a specific angle to the gradient of the height profile, preferably following this gradient. This can be used as an additional covert security feature, as it is invisible to the naked eye but can be detected during machine-assisted analysis of the security element.

[0017] Furthermore, it is possible to make the variation range of the vertical extension dependent on the tilt angle of the mirror surface. This can be done with a constant mirror length or with a variation along the gradient of the mirror surface.

[0018] The nanostructures can be described using the following parameters:

[0019] The elevations / depressions can be arranged regularly or irregularly, which means a quasi-statistical distribution of the elevations / depressions across the base area. In a regular arrangement, the grid structure used is a relevant parameter, for example, square grids, hexagonal grids, etc.

[0020] The spacing between the structures, i.e., the elevations / depressions, affects the color effect. In the case of irregularly arranged elevations / depressions, this applies to the average spacing and / or area coverage of the elevations / depressions on the base surface relative to the remaining areas of the base surface.

[0021] The lateral dimensions of the elevations / depressions further characterize the embossed layer. The elevations / depressions can be rotationally symmetrical in plan view, but can also extend along a single direction. This direction can be constant, but can also vary. The vertical extension of the elevations or depressions perpendicular to the base surface is another parameter that particularly affects the resulting color.

[0022] The basic shape of the elevations / depressions, i.e., their appearance in plan view, characterizes the embossed structure, as does the flank shape, i.e., the profile of the elevations / depressions in a cross-section perpendicular to the base. In plan view, basic shapes can be square, hexagonal, circular, elliptical, rectangular, etc. In section, the profile can have a sinusoidal shape, a parabola, a rectangular structure with approximately vertical flanks, etc.

[0023] The regions can particularly preferably be pixels of a displayed image or motif. Particularly preferably, the regions form pixels that are preferably arranged in a regular pattern (e.g., a square or hexagonal grid) and create the colored motif. This applies to all further developments or options described here.

[0024] After applying the reflector layer, the nanostructures can be coated with a dielectric material. The dielectric material covers the depressions / elevations, which are preferably formed in an embossing lacquer layer. The refractive index of the dielectric material is preferably identical to that of the embossing lacquer layer, but can also be different.

[0025] 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 restrictive. 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 exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary 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:

[0026] Fig. 1 shows a banknote with a security element, Fig. 2 shows a sectional view through the security element of Fig. 1, in a first embodiment,

[0027] Fig. 3 is a similar sectional view of a second embodiment,

[0028] Fig. 4 is a diagram explaining the operation of the embodiments,

[0029] Fig. 5 is a plan view of the security element of the second embodiment and

[0030] Fig. 6 an SEM image of this security element.

[0031] Fig. 1 shows a top view of a valuable document, in this case a banknote 2, which has a security element 4 intended to protect against counterfeiting. The security element 4 shows a colored image in top view. For this purpose, it has a structured surface, which can be seen in the sectional view in Fig. 2.

[0032] The security element is constructed on a substrate 6, on which there is an embossing lacquer layer 8 into which a nanostructure 10 is embossed. A preferred alternative to embossing will be explained below.

[0033] The nanostructure 10 has a plurality of depressions 12 and is provided with a reflector layer 14. Such nanostructures are generally known to those skilled in the art. They produce a color when viewed from above. The security element has at least two regions 16, 18 that differ with respect to the nanostructure 10, specifically with respect to the vertical extension of the depressions 12 relative to a base area 20, relative to which the depressions 12 are recessed.

[0034] In region 16, the vertical extension, i.e., the depth t of the depressions 12, increases along a direction 22. In region 18, the depth t varies along the direction 22 according to a different function, namely an approximately sinusoidal function.

[0035] The depth t of each depression 12 influences the color effect that appears in plan view. Since different depths are arranged along the direction 22 in region 16, a mixed color is created, as will be explained below with reference to Fig. 4. Since a different function is used in region 18, a different mixed color is created.

[0036] The mixed color between regions 16 and 18 also differs in the exemplary embodiment because the depth ranges are different. In region 18, the depressions 12 are significantly shallower than in region 16. This results in a mixed color 18 that is composed of different wavelengths with the same hue than in region 16, where deeper depressions 12 are used.

[0037] Fig. 2 shows, purely by way of example, the formation of the nanostructure 10 with depressions that are sunk downwards relative to the base area 20 (relative to the illustration in Fig. 2). A configuration in which elevations are arranged is equally possible. As a vertical extension, the height of these elevations is then naturally varied. A gradient gradient is particularly preferred for varying the vertical extension along direction 22. Such a gradient allows for a continuous mixing of colors that are close to one another in the color space. The shape of the gradient, for example, linear or quadratic, allows for the mixing ratio of the individual wavelengths of the same hue to be adjusted. Likewise, the distance between the depressions 12 and elevations can also be varied. This also influences the resulting color.

[0038] A particularly advantageous combination results from the nanostructure 10 when it is combined with a microstructure 24. This is shown schematically in Fig. 3. Here, the microstructure 24 is shown as a sequence of tilted micromirrors 25a-25d, with the individual micromirrors 25a-25d differing in their inclination relative to a ground plane, which can be defined, for example, as the surface of the substrate 6 or a plane parallel thereto. The nanostructure 10 is formed on each micromirror 25a-25d, which represents a microstructure element, and is then covered with the reflector layer 14. Fig. 6 shows an SEM image of the microstructure 24.

[0039] This results in a combination of the effects created by the microstructure 24 and the coloring in each individual microstructure element. In this way, for example, a bulging effect created by the microstructure 24 can be supplemented by a color effect that goes beyond uniform coloring, because the individual microstructure elements can create an individual mixed color, depending on how the function along the direction 22 is selected. This function is particularly preferably a gradient that is applied such that it follows the shape of the microstructure, i.e. the gradient direction 22 corresponds to a surface profile of the microstructure 24, for example the inclination direction of the micromirrors 25a-25d. In the illustration in Fig. 3, the direction 22 thus runs as in Fig. 2 and the sectional illustration in Fig. 3 shows the sectional plane in which the gradient of the mirror inclination lies. It is shown schematically as 25e. Fig.Figure 3 shows the simplified case where all micromirrors 25a-25d have a gradient 25e in the same direction, i.e., they are all tilted in the same direction. This is, of course, not mandatory. Each microstructure element can have its own gradient direction. The direction 22 of the nanostructure 10 on the corresponding microstructure element is then adapted to this gradient direction, in particular, aligned in the same direction.

[0040] The registration of the inclination gradients of the microstructure elements, e.g., the gradient 25e of a micromirror 25d, can be used as a hidden security element. Steeply positioned micromirrors, i.e., microstructure elements with a large height variation, then exhibit a large gradient in the nanostructure, while microstructure elements with a small height variation exhibit a smaller variation in the height of the depressions 12 or elevations. The following values ​​can be used as examples: In variant A this results in a depth variation T of 200 nm, in case B a depth variation of only 100 nm.

[0041] To create a bulge effect, the mirrors 25a to 25d can, for example, be arranged with a slight incline for the micromirror 25d, for example, between 0 and 5°, up to a large incline on the micromirror 25a, for example 30°. This is achieved by the nanostructure 10, which creates a mixed color in the yellow range, for example, by forming the elevations or depressions in a hexagonal arrangement with a grating period of 280 nm, an edge length of a square cross-section of the depressions / elevations of 140 nm, and a minimum depth of 80 nm. The variation in the vertical extent of the depressions 12 increases to different degrees starting from the minimum depth value of 80 nm, depending on the angle of inclination of the micromirror. Thus, for example, more steeply positioned mirrors display darker yellow tones, whereas flatter mirrors produce lighter yellow tones. This reinforces the plasticity of a three-dimensional appearance of the bulge effect.

[0042] This is illustrated in Fig. 5. Here, the vertically positioned mirrors are located at the edge labeled 26, whereas the flat mirrors are located in the center of the respective number, labeled 28. Likewise, the mixed color in area 26 is darker than in area 28. This results in the following values, for example:

[0043] Of course, these ratios can also be reversed, and a large gradient of depth variation can be used on flat mirrors and a smaller gradient on steep mirrors. This depends on the design and the mixed color to be created. Of course, the depth range can also be chosen differently on the individual mirrors, so that the mixed color differs more significantly, for example, in terms of the same hue wavelength.

[0044] Figs. 3 and 6 show a microstructure 24 using micromirrors 25a to 25d as an example. Of course, other microstructures, such as concave mirrors, Fresnel structures, lens structures, pillow structures, etc., can also be used, whereby the local gradient, i.e., the height profile, of the microstructure elements can also determine the direction of the gradient for the depth variation of the nanostructures.

[0045] A further advantage of arranging nanostructures 10 on microstructures 24 is that production fluctuations can be compensated for. An error in the imprinting of the nanostructures 10 can result in undesirable local changes in their depth. By varying the depth and creating a mixed color, such changes are less noticeable, since the mixed color caused by errors in the imprinting of individual depressions / elevations is less significant.

[0046] The effect of depth variation is shown in Fig. 4. There, a curve 30 plots the dependence of the chromatic wavelength X on the depth t of the depressions 12. The same would also apply to the height of elevations. Varying the depth over a depth variation range T automatically results in a wavelength mixture L. For clarity, Fig. 4 suggests a linear relationship. However, no axis scales are shown, as the relationship is nonlinear. Flatter structures do not automatically lead to shorter wavelengths. Experimental studies show that with increasing structure depth, different colors or "chromas" are covered:

[0047] In example A, the color changes from shallow to deep structures, i.e. with increasing structure depth, from pale yellow to rich yellow to gold.

[0048] In example B, the color changes from shallow to deep structures, i.e. with increasing structure depth, from orange to magenta to violet and blue to green.

[0049] As mentioned, the security element 4 can be manufactured by embossing. Alternatively, photolithographic production is used, which takes advantage of local sensitivity differences in a photoresist that arise due to microstructuring.

[0050] To achieve the different vertical extensions of the depressions and elevations of the nanostructures, the fact that the exposure for the microstructure 24 has already taken place is exploited. This automatically prevents all depressions of the nanostructure 10 from being designed with the same vertical extension. The exposure of the microstructure already exposed an illumination dose gradient (corresponding to the height profile of the microstructure elements) into the photoresist. A comparatively small dose was applied to the upper edge of each mirror, while more dose was introduced into the resist at the lowest points of the mirror to achieve the mirror inclination. The photoresist is therefore increasingly bleached away from the upper edge of the mirror; the bleaching increases towards the lowest points of the height profile.When the nanostructure is exposed, the resist, which has already faded to varying degrees, reacts differently to the same exposure dose, depending on how much dose was previously applied to create the microstructure 24. This makes it possible to work with a uniform exposure dose for the recesses 12 of the nanostructure 10 and yet still obtain recesses 12 of varying depths. The depth is proportional to the dose already applied to create the microstructure 24 and is thus automatically registered to the height of the microstructure.

[0051] For a different variation in the depth of the nanostructures, a dose wedge is used for the exposure of the nanostructures 10 to specifically vary the depth. This dose wedge must take into account the dose of the first exposure, i.e., the exposure to the microstructure 24, as well as the shape and direction of the gradient to be realized for the vertical extension.

[0052] As an alternative to a photolithographic realization, the embossing already mentioned in Figs. 2 and 3 can be carried out in an embossing lacquer 8.

[0053] After the reflector layer 14 has been applied, the nanostructures 10 are preferably overcoated with a dielectric material (not shown). The dielectric material covers the depressions / elevations so that the nanostructure 10 is completely or partially leveled. This increases the durability and resistance to contamination of the security element. In addition, the color effect can be further controlled, since the refractive index of the dielectric material affects the color. The refractive index of the dielectric material is preferably identical to that of the layer in which the nanostructures 10 are formed, e.g. the embossing lacquer layer 8, but can also be different. In the case of nanostructures 10 that are located on a flat base structure (as in Fig. 2), the overcoating with the dielectric material can be carried out by applying and abrading no excess material. In the case of nanostructures 10 that are formed on inclined mirrors (as in Fig.4) a deposition process for applying the dielectric material can be considered.

[0054] Reference list

[0055] 2 banknotes

[0056] 4 Security element

[0057] 6 Substrat

[0058] 8 embossing lacquer layers

[0059] 10 Nanostructure

[0060] 12 Deepening

[0061] 14 Reflector layer

[0062] 16, 18 area

[0063] 20 floor space

[0064] 22 direction

[0065] 24 Microstructure

[0066] 25a-d Micromirrors

[0067] 25e gradient

[0068] 26, 28 area

[0069] 30 Curve t Depth z Hue-like wavelength

[0070] T Depth variation

[0071] L Wavelength mixing

Claims

P a t e n t a n s p r ü c h e 1. A security element comprising a structured layer (8) having a base area (20) and, relative to the base area, recesses (12) or raised elevations, and a reflector layer (14) arranged on the structured layer (8), wherein the recesses (12) or elevations are formed as color-generating nanostructures (10) with regard to their extents along the base area (20), their vertical extent (t) perpendicular to the base area (20) and their arrangement on the base area (20), wherein the structured layer (8) has a plurality of regions (16, 18; 25a-d) providing a colored motif or image, wherein the regions (16, 18; 25a-d) preferably form pixels, characterized in that in each of the plurality of regions (16, 18; 25a-d), the vertical extent (t) of the recesses (12) or elevations along a direction (22) varies according to a non-constant function, so that in each of the plurality of regions (16, 18;25a-d) in plan view a colour is visible which is created as a mixed colour by varying the vertical extension; 2. Security element according to claim 1, characterized in that at least some of the regions (16, 18; 25a-d) differ in terms of function.

3. Security element according to one of the above claims, characterized in that the vertical extension of the depressions (12) or elevations along the direction (22) increases or decreases according to a gradient (25e).

4. Security element according to one of the above claims, characterized in that in the regions the depressions (12) or elevations have the same extents along the base surface (20) and / or arrangement on the base surface (20).

5. Security element according to one of the above claims, characterized in that the regions (25a-d) are each formed as elements of a microstructure (24), in particular as inclined mirror surfaces in which the base surface (20) is tilted relative to a base plane, wherein at least some of the regions (25a-d) differ from one another with regard to the tilt and / or shape of the mirror surface.

6. Security element according to claim 5, characterized in that the direction (22) in each region (25a-d) follows a gradient (25e) of the elements of the microstructure (24), in particular the mirror surface.

7. Security element according to claim 5 or 6, characterized in that the vertical extension (t) increases or decreases with the height of the mirror surface above the ground plane.

8. Security element according to one of claims 5 to 7, characterized in that the microstructure (24) causes a bulging effect.

9. A method for producing a security element, the method comprising Producing a structured layer (8) which has a base surface (10) and, relative to this, recessed depressions (12) or raised elevations, and Arranging a reflector layer (14) on the structured layer (8), wherein the depressions (12) or elevations are formed as a color-generating nanostructure (10) with regard to their extents along the base surface (20), their vertical extension (t) perpendicular to the base surface (20) and their arrangement on the base surface (20), the structured layer (8) is formed with a plurality of regions (16, 18; 25a-d) which provide a colored motif or image, wherein the regions (16, 18; 25a-d) preferably form pixels, characterized in that in each of the plurality of regions (16, 18; 25a-d) the vertical extension (t) of the depressions (12) or elevations varies along a direction (22) according to a non-constant function, so that in each of the plurality of regions (16, 18; 25a-d) in plan view a color which is determined by the variation of the vertical Extension as a mixed color arises, becomes visible.

10. Method according to claim 9, characterized in that a security element is produced according to one of claims 1 to 8.

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