Security element with reflective surface area, data carrier and method for producing same

Abstract

The present invention relates to a security element for providing security for valuable articles, which has a reflective surface area, which extends to define an x-y plane and produces a multi-color three-dimensional view when observed with reflected light. The reflective surface area includes a coarse structure that produces a three-dimensional view and a fine structure superimposed on the coarse structure for producing a multi-color appearance of the view. The coarse structure includes a plurality of directional reflective segmented mirrors, which are oriented so that the reflective surface area can be perceived by an observer in the form of a three-dimensional view, and the three-dimensional view has a surface that protrudes and / or is recessed relative to its real spatial shape. The fine structure is formed by a stepped Bragg interference structure with metallized steps, which are superimposed on the directional reflective segmented mirror of the coarse structure and produce a color effect by Bragg interference at adjacent steps when observed. The locally different heights of the steps in the reflective surface area produce locally different colors of the three-dimensional view.

Description

Technical Field

[0001] The invention relates to a security element for providing security for valuable articles, the security element having a reflective surface area, the extension of which defines an xy plane and which produces a multi-colored three-dimensional image when viewed in reflected light. The invention also relates to a data carrier having such a security element and a method for producing such a security element. Background Art

[0002] Data carriers such as valuable documents or ID documents or other valuable articles such as brand-name goods are often provided for security purposes with security elements which can verify the authenticity of the data carrier and at the same time serve as protection against unauthorized copying. Security elements with an angle-dependent or three-dimensional appearance play a particular role in authenticity assurance, since such security elements cannot be reproduced even with the latest copying equipment.

[0003] Patent document WO 2014 / 060089 A2 proposes to generate a three-dimensional camber effect in a security element by means of micro-mirrors. The micro-mirror structure can be easily reproduced by a skilled hand, for example, by means of an embossing method. The embossed relief structure is metal-coated and embedded in plastic. However, its appearance is usually only monochrome in the case of a metal coating.

[0004] In order to also produce colored views, instead of simple metallization, the micromirrors can be provided with an angle-dependent coating which produces a color dependency of the structure depending on the viewing angle. However, a laterally different color design of the pattern cannot be achieved in this way.

[0005] In other methods, metal nanostructures are used, which show a colored appearance in reflection or transmission. In particular, subwavelength gratings can be used to produce color effects with the finest spatial resolution. For this purpose, patent document DE102011101635A1 describes an embossed structure, which has a two-dimensional periodic nanostructure, which shows color in reflection and transmission. Here, the embossed structure only needs to be evaporated with an aluminum layer. With this nanostructure, not only can the entire color be colored, but also different local colors can be produced in the pattern (Motiv, or visual object).

[0006] For mass production, the nanostructures used must be able to be formed with high structural stability and low defect rates. However, nanostructures used for color effects to date usually have an aspect ratio of more than 1 and a period of less than 300 nm. Imprinting processes for defect-free mass production of such structures are technically very demanding and complex. Summary of the invention

[0007] Based on this, the technical problem to be solved by the present invention is to provide a security element of the type mentioned at the beginning of this article, which can also be simply and cost-effectively produced on the larger technical scale required in the field of anti-counterfeiting and has an attractive multi-color three-dimensional appearance in addition to a high degree of anti-counterfeiting security.

[0008] This object is achieved according to the invention by a security element for securing a valuable object, a data carrier having a security element and a production method for a security element.

[0009] According to the invention, in a security element of the type described in the invention, it is provided that the reflective surface area comprises a coarse structure for generating a three-dimensional image and a fine structure superimposed on the coarse structure for generating a multi-colored appearance of the image.

[0010] In this case, the coarse structure comprises a plurality of directionally reflective segmented mirrors (Facettes) which are oriented such that the reflective surface areas can be perceived by an observer in the form of a three-dimensional image having a surface that is protruding and / or recessed relative to its actual spatial shape.

[0011] The fine structure is formed by a stepped Bragg interference structure with steps, on which a reflective or reflection-enhancing coating is provided, wherein the steps are superimposed on a coarse structure of a directionally reflective segmented mirror and when observed, the steps produce a color effect by Bragg interference at adjacent steps. In this case, the locally different heights of the steps produce locally different colors of the three-dimensional image in the reflective surface area.

[0012] In an advantageous embodiment, the coarse and fine structures of the reflective surface area are embedded between transparent layers, in particular transparent dielectric layers. The dielectric layer advantageously has a refractive index between 1.3 and 1.7, in particular approximately 1.5. The refractive index of the layer in which the coarse and fine structures are embedded is advantageously substantially the same and is, for example, approximately 1.5.

[0013] The coarsely structured segmented mirror preferably has a width of between 2 μm and 30 μm, in particular between 4 μm and 10 μm, in at least one spatial direction of the xy plane. Advantageously, the segmented mirror even has a width in the stated range in both spatial directions of the xy plane, wherein the width in both spatial directions does not necessarily have to be the same. The distance between adjacent segmented mirrors usually corresponds to the width of the segmented mirror. In this case, the distance between adjacent segmented mirrors can be periodic, but it is particularly advantageous if the coarsely structured segmented mirrors are arranged aperiodically in order to avoid undesired grating diffraction effects.

[0014] The steps superimposed on the coarse structure advantageously have the same height in each segmented mirror in order to produce a bright and well-defined color effect. As explained in detail below, the color effect of the Bragg interference structure is essentially determined by the height of the steps, so that sub-areas with different color effects usually contain steps of different heights.

[0015] In an advantageous embodiment, a Bragg interference structure having 2 to 20 steps, in particular 2 to 10 steps, is superimposed on the coarsely structured segmented mirror in the colored region.

[0016] The height of the steps is preferably between 100 nm and 600 nm, particularly preferably between 125 nm and 520 nm and in particular between 150 nm and 425 nm.

[0017] In an advantageous embodiment, it is provided that in the case of flatter segmented mirrors, whose slope is less than a predefined threshold value, the height of the step is less than 275 nm, and in the case of steeper segmented mirrors, whose slope is greater than the threshold value, the height of the step is greater than 275 nm. In this way, the number of required step steps can also be kept small in the case of steeper segmented mirrors. The predefined threshold value for the slope of the segmented mirror depends primarily on the desired color.

[0018] As explained in detail below, in a suitable design, the same color effect can be produced by a higher order constructive interference with a larger step height (or step height) as by a first order constructive interference with a lower step height. Here, the inventors have realized that in the case of an embedded dielectric having a refractive index of n≈1.5, all desired colors can be obtained by a first order constructive interference with a step height of less than 275 nm, while the same colors can be obtained by a second or higher order constructive interference with a step height of greater than 275 nm. Thus, the same color effect can be obtained by a smaller step height in a flatter segmented mirror and by a larger step height in a steeper segmented mirror, so there is no significant difference in the number of steps required in the flatter and steeper segmented mirrors.

[0019] The step advantageously encloses an angle α between 85° and 120° with the z-axis, in particular an angle α not equal to 90°. The angle α can advantageously be in the range of 100° to 110°. The choice of an angle α not equal to 90° provides an additional degree of freedom when adjusting the color intensity (or brightness).

[0020] The reflective or reflection-enhancing coating can be particularly advantageously a vapor-deposited metallization, for example. The stepped metallization is advantageously made of Al, Ag, Au, Ni, Cr or an alloy of these metals. The thickness of the metallization is preferably between 8 nm and 150 nm, in particular between 15 nm and 60 nm.

[0021] Instead of a metallization, the stepped reflection or reflection-enhancing coating can also be formed by a coating of a material with a higher refractive index. In particular, ZnS, ZnO, ZnSe, SiN x 、SiO x 、Cr2O3、Nb2O5、Ta2O5、Ti x O x Or ZrO2. It is particularly advantageous here if the security element is provided with an absorbing matrix. When viewed, a structure provided with a highly refractive coating shows an optical effect similar to a structure with a coating of liquid crystal material, but compared to the latter, it has the advantage that the color tone is defined by steps and can be freely selected in the transverse direction.

[0022] Alternatively, the reflective or reflection-enhancing coating can also be formed from a semimetal, in particular from Si or Ge.

[0023] The width of the steps is preferably between 400 nm and 2 μm, in particular between 600 nm and 1 μm. The width of the steps is preferably the same in each segmented mirror in order to ensure efficient light interference and thus a high brightness of the reflection.

[0024] Advantageously, the stepped Bragg interference structure produces in the colored region of the reflective surface region a chromaticity C* in the LCh color space of greater than 40, in particular greater than 60 or even greater than 80. Such high chromaticity values ​​can be achieved by suitable selection of the height of the steps and provide a color impression with a high color saturation.

[0025] The view generated by the reflective surface area preferably has three or more, in particular five or more, different colored colors. The view can in particular be a true color view, i.e., a pattern with a plurality of true colors is presented. In this context, a mixed color consisting of a maximum of three primary colors, such as red, green and blue, is generally referred to as a true color. The multi-color view can also advantageously have motion effects.

[0026] The invention also encompasses a data carrier having a security element of the type described. The data carrier can be, in particular, a valuable document such as a banknote, in particular a paper banknote, a polymer banknote or a film composite banknote, a share certificate, a bond, a certificate, a coupon, a check, a stamp, a tax stamp, a high-value ticket, but also an ID card such as a credit card, a bank card, a cash card, an authorization card, an identity card or a personal page of a passport.

[0027] Finally, the invention also provides a method for producing a security element of the type described, in which method:

[0028] - generating a reflective surface area having a coarse structure for generating a three-dimensional image and a fine structure superimposed on the coarse structure, the fine structure for generating a multi-colored appearance of the image,

[0029] - the coarse structure is designed as a segmented mirror with a plurality of directional reflections, the segmented mirrors being oriented in such a way that the reflective surface areas can be perceived by an observer in the form of a three-dimensional image with surfaces that are protruding and / or recessed relative to its real spatial shape, and

[0030] -The fine structure is formed by a stepped Bragg interference structure with steps, which are superimposed on a coarse structure of a directionally reflective segmented mirror and which, when observed, produces a color effect due to Bragg interference at adjacent steps, wherein the steps are locally designed to have different heights in order to produce locally different colors of a three-dimensional view in the reflective surface area, and wherein a reflective or reflection-enhancing coating is designed on the steps.

[0031] The proposed security element is based on a novel superposition of a coarse structure consisting of directional reflective segmented mirrors and a fine structure consisting of a stepped Bragg interference structure. The security element displays a spatial effect when viewed and appears colored due to the superimposed Bragg interference structure. By selecting the step heights, different colors can be produced in a targeted manner in a three-dimensional view. Due to the low aspect ratio of the Bragg interference structure, the reproduction of an embossed structure with such a coarse and fine structure can be easily and cost-effectively realized.

[0032] The coloring is achieved based on Bragg interference at steps of different heights, so that even with a simple metal coating, such as a metal coating made of aluminum, a coloration or chromaticity is already formed. As a result, mass production is significantly more cost-effective than structures that require multi-layer coatings, such as multi-layer coatings that vary in color with angle, to achieve their color effects. In addition, colored patterns with the proposed structure can be achieved with the highest resolution.

[0033] The described segmented mirrors and Bragg interference structures can also be easily combined with other known embossed structures, such as holograms or other nanostructures, and can already be arranged side by side on an embossing punch with high precision. In this way, structures with different optical effects can be perfectly registered with each other and, for example, spatial effects, motion effects and multi-color patterns can be combined in a single security element.

[0034] For example, by combining the structures described herein with light-absorbing imprinted structures (such as moth-eye structures), high-contrast patterns and true-color images can be produced. In particular, the contrast of a pixelated true-color image formed by a Bragg interference structure can be adjusted by selecting the area ratio of the light-absorbing structure in each pixel. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Exemplary embodiments and advantages of the invention are explained below with reference to the drawings, in which the figures are not shown to scale or to proportion in order to improve the clarity.

[0036] In the attached picture:

[0037] Figure 1 A banknote is schematically shown which is provided with two security elements according to an exemplary embodiment of the present invention;

[0038] Figure 2 A schematic cross-sectional view of a detail of the security element according to the invention and a perspective view of the three-dimensional view perceived by an observer when observing the security element are shown above the security element;

[0039] Figure 3(a)-3(b) To illustrate the principle of color generation, Figure 3(a) shows Figure 2 3( b ) shows a comparison of two sub-areas in which the simulated surface has the same slope but appears with different color impressions;

[0040] Figure 4 A top view with contour lines of a curved surface to be simulated is shown, and the orientation of the segmented mirrors according to the local gradient of the surface is shown by way of example at three positions;

[0041] Figure 5 The position of the steps of the Bragg interference structure in the security element is shown, the steps being formed by a metal coating around a homogeneous dielectric;

[0042] Figure 6 (a) and (b) show spectral curves of reflection at a four-step structure having steps with different step heights;

[0043] Figure 7 shows the CIE color diagram, which has Figure 6 The color coordinates of the reflectance spectrum;

[0044] Figure 8shows the color properties of a stepped Bragg interference structure with 4 steps, wherein (a) the brightness L*, (b) the chromaticity C* and (c) the hue h° are shown in the LCh color space as a function of the node height h of the segmented mirror, and (d) the change in the color coordinates in the CIE color diagram with increasing node height;

[0045] Fig. 9 Shown as Figure 6 (b) Reflection spectrum of a Bragg step structure with the same parameters as in (b) but with a smaller width;

[0046] Fig.10 In (a) and (b) are shown the spectral curves of the reflection at a 9-step stepped structure having different step heights and different orders of constructive interference;

[0047] Fig.11 In (a) and (b) are shown the spectral curves of the reflection at the step structure for different metal coating thicknesses;

[0048] Fig.12 Spectral curves showing reflection for different light incident angles;

[0049] Fig.13 shows the angular dependence of the color impression, wherein (a) the color coordinates at light incidence angles of θ=0° and θ=20° are compared in the CIE color diagram, and (b) the lightness L* is compared for the two light incidence angles in the LCh color space, (c) the chromaticity C* is compared, and (d) the hue h° is compared; and

[0050] Fig.14 (a) and (b) schematically illustrate intermediate steps in the production of the security element according to the invention. DETAILED DESCRIPTION

[0051] The present invention will now be described using the example of a security element for banknotes. Figure 1 A schematic diagram of a banknote 10 is shown, which is provided with two security elements 12, 16 according to an exemplary embodiment of the present invention. The first security element is a glued-on transfer element 12, which displays a multi-colored, three-dimensional visual appearance when viewed. Fig.14 , the view is in the form of a true color portrait that appears to arch out from the plane of the banknote.

[0052] The second security element is a security thread 16 which displays the denomination of the banknote 10 alternately along its length and as a graphic pattern displays multi-colored stars 18 which appear to be curved out of the plane of the banknote. However, it goes without saying that the invention is not limited to the patterns shown by way of example or to transfer elements and security threads on banknotes, but can be applied to all types of security elements, for example in labels on goods and packaging or for the security of documents, ID cards, passports, credit cards, health cards, etc.

[0053] The security element according to the invention can also be used to realize a pattern over the entire surface on a security document, for example in a banknote. This method is described in detail in DE 10 2014 018 551 A1. By using a structure of the type described here, the use of pigment inks can be avoided when manufacturing security documents and the number of process steps when manufacturing such security documents can be reduced.

[0054] Now refer to Figures 2 to 4 The formation of a multi-color three-dimensional image in the security element according to the invention is explained in more detail. First, Figure 2 The basic formation of a three-dimensional appearance in a security element 20 according to the invention is described. The drawing schematically shows a detail of a security element 20 according to the invention in cross section and shows a perspective view 40 of the three-dimensional view perceived when viewing the security element above the security element.

[0055] The security element 20 comprises a reflective surface area 22 having a relief structure 28 which is embossed in an embossing lacquer 24 and provided with a reflective or reflection-enhancing coating, in particular a metallization 26. The relief structure 28 is a superposition of a coarse structure 30 and a color-generating fine structure 50 (see FIG. 3 ), but substantially only the coarse structure is responsible for the three-dimensional appearance, so that in Figure 2 Only the coarse structure is shown in the simplified view.

[0056] The extension of the reflective surface area 22 defines an xy plane, the direction perpendicular to which is referred to as the z direction. The security element 20 is designed for observation from the positive z direction, wherein a typical observation distance is about 30 cm. The coarse structure 30 comprises a plurality of directionally reflective segmented mirrors 32, which are each oriented such that the reflective surface area 22 can be perceived by an observer in the form of a desired three-dimensional image 40, for example Figure 1 The three-dimensional representation, such as the human figure 14 or the curved star 18 , has surfaces that are protruding and / or recessed relative to the actual spatial shape of the surface area 22 .

[0057] The metallized embossing lacquer layers 24, 26 are embedded in dielectric layers 34, 36, 38, the refractive index of which can all be the same and is typically approximately n=1.5. Figure 2 As can be seen in FIG. 1 , the security element 20 is not designed with the curved surface 40 itself perceived by the observer, but with a plurality of reflective segmented mirrors 32 which imitate the reflection behavior of the curved surface 40 by their orientation, but have a significantly smaller node height than the imitated curved surface. Since the segmented mirror 32 acts like a small micromirror, the segmented mirror array is often also referred to as a micromirror array and the segmented mirrors are referred to as micromirrors within the scope of this specification.

[0058] For example, the reflective segmented mirror 32 is designed with a rectangular outline and a width W between 4 μm and 10 μm. Thus, each segmented mirror 32 of the coarse structure 30 forms a planar element which is tilted at a certain angle relative to the xy plane, wherein the tilt angle corresponds exactly to the local slope of the simulated surface 40. The segment height h of the segmented mirror 32 That is, the height difference of the segmented mirror relative to the xy plane is between 0 μm and 10 μm, so the actual height difference in the reflective surface area 22 cannot be perceived by the naked eye.

[0059] The width and the section height of the coarse structured segmented mirror 32 can be selected in different ways. For example, the desired section height h can be preset and the width W of the segmented mirror 32 can be selected in such a way that the desired slope of the segmented mirror 32 is generated in combination with the preset section height. For the combination with the fine structure that produces color as described in detail below, it has proven to be advantageous to preset the width W of the segmented mirror and select the section height h in such a way that the desired slope of the segmented mirror 32 is generated. Here, the width W of the segmented mirrors is selected to be essentially the same size. However, it is advantageous that the distance of the segmented mirrors is slightly changed in an irregular manner to avoid undesirable grating diffraction effects due to a completely regular arrangement.

[0060] At least the most inclined segmented mirror has a section height h in profile that is significantly larger than the wavelength (>1 μm). The slope of the steepest segmented mirror (i.e. the segmented mirror with the largest slope relative to the xy plane) is typically in the range of 20°, in special designs even in the range of 10°.

[0061] In order to generate not only a three-dimensional image, but also a multi-color three-dimensional image according to the invention, the coarse structure 30 is superimposed with a color-generating fine structure in the form of a stepped Bragg interference structure. In order to illustrate the principle of color generation, FIG. 3( a) shows the relief structure 28 of the security element 20 in more detail in two sub-areas 20-A, 20-B in which the simulated surface 40 and therefore the segmented mirror 32 have different slopes, but in both sub-areas the same color impression is generated when viewed.

[0062] In the present exemplary embodiment, the fine structure 50 superimposed on the coarse structure of the segmented mirror 32 consists of an array of metallized step levels 52, each of which has a width w i and height h i For each segmented mirror 32 (or mirror segment), the step width w of the step level superimposed with the segmented mirror is i The sum of forms the width of the segmented mirror, and the step height h i The sum of forms the segment height h of the segmented mirror.

[0063] The color effect of the sub-regions 20 -A, 20 -B is produced by the Bragg interference of incident light at adjacent step levels and is substantially determined by the height h of the step level. i More precisely, as long as the wavelength λ of the light satisfies the condition

[0064] λ / n=2h i / m*sin(90°-θ)(IB)

[0065] Constructive interference occurs at adjacent step levels 52 , where n is the refractive index of the medium, θ is the incident angle of the light relative to the z direction and m is an integer greater than or equal to 1.

[0066] The above relationship is based on a wave optics model for interference, which is valid for sufficiently large step steps. For smaller and smaller sizes in the subwavelength range, diffraction no longer occurs according to this classical model and must be calculated according to Maxwell's equations. Thus, in the case of individual step widths in the subwavelength range (λ<400nm), the step steps no longer act like individual mirrors with respect to the incident light.

[0067] If a sub-region reflects a color with a centroid wavelength λ at normal incidence (θ=0°), the steps in the sub-region are designed to have a height h according to the relation (IB). i The height corresponds substantially to half the wavelength of the light in the medium, λ / (2n), or to an integer multiple of this wavelength, ie m*λ / (2n). Advantageously, the steps are designed for m=1 or m=2.

[0068] Here, the width w of the step i It is expedient to select such that adjacent surface ratios are substantially of the same size, since this ensures the best possible light interference at adjacent step levels. i For a flatter segmented mirror 32, it is advantageous to use steps whose height h is icorresponds to half the wavelength of the light in the medium (ie m=1 in relation (IB)), while for steeper segmented mirrors 32 it is advantageous to use steps whose height h i corresponds to a multiple of half the wavelength of the light in the medium (in particular m=2 in relation (IB)), since for example at m=2 and the step width w of each step i In the same case, the vertical distance spanned is twice that of a step with m=1.

[0069] The height of the steps is adjusted in a suitable manner relative to the inclination of the individual segmented mirrors so that the desired color is achieved with the greatest possible intensity. Fig.14 As can be seen from the view of , when the height increases, the bottom area or area ratio of the step increases significantly (for the same color, for example, doubled or multiplied). i As the step width w decreases, the (desired) interference ratio of the light decreases and the scattering ratio increases. i As the slope increases, the proportion of light reflected by the mirror increases. In between, for a specific slope, there is an optimum value for the light intensity with respect to the interference at the step.

[0070] In order to obtain the best interference conditions, the width of the step is preferably in the range of 400 nm to 2 μm. For the step arranged parallel to the xy plane, the width w of the step is i By w i =h i / tan(β), where β corresponds to the tilt angle of the segmented mirror relative to the horizontal and h i Corresponds to the height of the steps.

[0071] Therefore, for a surface inclined at 45°, the width of the step w i and height h i are equal. To form blue, the step height h i (and correspondingly the width w i ) is about 150nm. For the 15° tilted surface, the step height h is 140nm to 210nm (depending on the color). i This results in step widths of 522 nm to 784 nm. Therefore, in the case where the segmented mirror tilt angle is greater than about 20° (threshold), the step levels are usually designed for multiple values ​​of m (i.e., m=2 or greater in relation (IB)). However, depending on the desired color and the desired step width w i Depending on the choice of , this value can also vary by several degrees for different areas of the fine structure.

[0072] Returning to the view of FIG. 3( a ), the steps 52 in the two sub-regions 20 -A and 20 -B are exemplarily designed to have a height h i =175 nm, so when embedded in a dielectric 34, 36, 38 with a refractive index of n=1.5, a bright green color with a wavelength of approximately 525 nm is produced as the color reflected by the fine structure 50 according to the relationship (IB).

[0073] As shown in FIG. 3( a ), although in each sub-region 20 -A or 20 -B, the step width w of the segmented mirror 32 is i is constant, but due to the different slopes of the segmented mirrors 32 in the two sub-regions, the step width w i is different between the two sub-areas. Due to the step height h i Likewise, the step in the region 20 -B with the flatter segmented mirror has a greater width w than the step in the region 20 -A with the steeper segmented mirror. i .

[0074] FIG3( b ) shows a comparison of a sub-region 20 -A with another sub-region 20 -C of the security element 20 , in which the segmented mirror 32 has the same slope as in the sub-region 20 -A but appears in a different color impression. In the sub-region 20 -C, the steps 52 of the fine structure are designed with a greater height h i , thus satisfying the interference condition for a larger light wavelength of relation (IB). Specifically, the step 52 in the sub-region 20-C is designed to have a height h i =210 nm, so a bright red color having a wavelength of about 630 nm occurs as the reflected color.

[0075] 3 (a) and (b) as shown, for each sub-area of ​​the security element 20, starting from the slope of the segmented mirror 32 preset by the shape of the simulated surface at that location, by appropriately selecting the step width w i and level height h i to adjust the desired color for this sub-region.

[0076] Understandably, Figure 2 The illustrative view of FIG. 3 represents a strong simplification of the actual conditions. In the case of a general three-dimensional surface, the individual segmented mirrors 32 are oriented parallel to the local gradient of the surface to be simulated and perpendicular to the contour lines of the surface to be simulated. Figure 4 , which shows a top view of a curved surface 40 to be simulated with contour lines 42. The orientation of the segmented mirror 32 according to the local gradient 44 of the surface 40 is shown by way of example in three positions. Figure 2According to the design principle of FIG. 4 , the local gradient 44 corresponds exactly to the local inclination of the segmented mirror 32 relative to the xy plane of the security element 20 .

[0077] Additionally, Figure 4 The surface 40 in the embodiment of the present invention is divided into sub-areas 40-A, 40-B, 40-C, in which the curved surface appears in different colors when observed. The segmented mirror 32 is provided with correspondingly matched color-generating fine structures in the form of stepped Bragg interference structures with appropriately selected step widths w i and level height h i 3. Since the color effect for each segmented mirror can be adjusted individually, the surface 40 can be displayed with high-resolution color effects and sharp color transitions.

[0078] Now refer to Figures 5 to 13 Further properties and advantages of the combination according to the invention of coarse and fine structure are described in more detail.

[0079] Figure 5 First, the possible orientations of the steps 52 of the Bragg interference structure 50 in the security element 20 relative to the z-direction are explained. In the case of the interference structure 50-A shown in the left half of the figure, the steps 52 are arranged parallel to the xy plane, so that the angle α between the steps and the z-direction is α=90°. However, within the scope of the present invention, other angles can also be advantageously selected, in particular α can be between 85° and 120°. In the case of the interference structure 50-B shown in the right half of the figure, the angle α between the steps and the z-direction is, for example, α=100°. The choice of angle α provides additional degrees of freedom in adjusting the color intensity.

[0080] Variation of the angle α affects the formation of optical interference between adjacent steps by changing the path difference between the optical paths.

[0081] Therefore, the distance h of the Bragg plane is i In the relationship (IB) it changes to

[0082] h i '=t i *cos(90°-α)–w i *sin(90°-α)(1)

[0083] Where α is the angle between the step and the z direction, t i is the height of the step and w i is the width of the step.

[0084] This results in a modified relationship (IB) for “inclined” steps, ie for values ​​of α not equal to 90°, in which, in addition to the distance h of the Bragg plane i , the incident angle θ of the light is also corrected (θ'=θ-(90°-α)):

[0085] λ / n=2h i ' / m*sin(90°-θ')(IB1)

[0086] =2(t i *cos(90°-α)–w i *sin(90°-α)) / m*sin(θ+α)).

[0087] Thus, the examples of steeper segmented mirrors described below can advantageously be formed by steps with α>90° without having to select the step height h i multiples of (in relation (IB) m = 2 or greater).

[0088] For simpler display, the angle α can also be defined as an angle relative to the xy plane, so for a horizontally oriented step α applies xy =0°. Then the above relationship can be expressed by

[0089] λ / n=2(t i *cos(α xy )–w i *sin(α xy ) / m*sin(90°-θ+α xy ))(IB2)

[0090] Given.

[0091] exist Figure 6 The spectral curve of the reflection at a four-step structure of the type shown in FIG3 is shown in FIG4 , where it is assumed that the non-polarized light is incident vertically. The steps have a constant height h i Aluminum metal strips 26 with a layer thickness of s=40 nm are provided. The relief structures are respectively embedded in dielectrics 34, 36, 38 with a refractive index of n=1.5.

[0092] Figure 6 The diagram 60 of (a) shows the three different step heights h i In h i =140nm (reflection spectrum 62), h i =175nm (reflection spectrum 64) and h i= 210 nm (reflection spectrum 66) as a function of the wavelength λ. Here, the segmented mirrors 32 of the relief structure are arranged with a periodic distance of W = 6 μm and equal width. i , the reflection spectrum shows a clear maximum in the blue wavelength range (reflection spectrum 62 ), in the green wavelength range (reflection spectrum 64 ) or in the red wavelength range (reflection spectrum 66 ).

[0093] Figure 6 The diagram 70 of (b) shows that the step height h is approximately twice as large as i The reflectivity R of the convex structure, that is, h i =300nm (reflection spectrum 72), h i =360nm (reflection spectrum 74) and h i =400 nm (reflection spectrum 76). Here too, the relevant maxima of the reflection spectrum lie in the blue (450 nm), green (540 nm) or red (600 nm).

[0094] Figure 6 The reflection spectrum of (a) shows the first-order Bragg reflection at step 52 (m=1 in relation (IB)), while Figure 6 The maximum value of (b) is produced by the second-order Bragg reflection at the adjacent step, ie m=2 in relation (IB). A comparison of the reflection curves of the two diagrams 60 and 70 shows that even if the step height h i Doubling the value can also produce a color representation in the desired centroid wavelength through constructive interference. For the double-order height of m=2, the half-value width of the maximum value is even smaller than the half-value width in the single-order height of m=1.

[0095] To study in more detail Figure 6 The color characteristics of the structure are calculated by convolving the calculated reflection spectra 62-66 and 72-76 with the emission curve of the D65 standard lamp and the sensitivity of the human eye. The color coordinates X, Y, and Z calculated in this way are converted into standard color value components x and y and are calculated in Figure 7 The CIE color diagram 80 is shown.

[0096] Figure 6 The color coordinates of the reflection spectrum are indicated by the reference numerals 62', 64', 66' and 72', 74', 76' with ' respectively. The color coordinates given by the standard color value components can be directly assigned to the human perception when perceiving color. For orientation, the white point 82 of the color space 86 and the triangle 84 are also drawn, which defines the color space that can usually be displayed on the screen.

[0097] from Figure 7It can be directly seen from the comparison of the color coordinates of the reflection spectrum with the white point 82 and the screen triangle 84 that the stepped Bragg interference structure according to the present invention is very suitable for forming RGB colors with high color saturation. Both the structure with first-order Bragg reflection (m=1, color coordinates 62', 64', 66') and the structure with second-order Bragg reflection (m=2, color coordinates 72', 74', 76') can be used for color expression. i =360nm) produces a green hue that is even brighter than at one-fold height (h i =175 nm) has a higher saturation, which can be seen from the greater distance between the color coordinate 74 ′ and the white point 82 than the distance between the color coordinate 64 ′ and the white point 82 .

[0098] In order to further study the color characteristics of the Bragg interference structure according to the present invention when the step height is continuously changed, the reflection spectra were calculated for different segment heights h between 0.6 μm and 1.7 μm of the segmented mirror, and the color coordinates of the reflection in the CIE color space and the LCh color space were calculated from these reflection spectra.

[0099] Figure 8 The results are shown for a stepped, aluminum-coated Bragg interference structure with 4 steps, a width of W=6 μm and a metal layer thickness of s=40 nm, each embedded in a dielectric with n=1.5 and with vertical incidence of light. In this case, depending on the segment height h of the segmented mirror, Figure 8 (a) shows the calculated lightness L* as curve 90, Figure 8 (b) shows the calculated chromaticity C* as curve 92 and Figure 8 (c) shows the calculated hue h° as curve 94 .

[0100] As a function of increasing node height, Figure 8 The curve 96 of (d) shows the change of the color coordinates in the CIE color diagram. The starting point of the curve 96 at h = 0.6 μm is indicated by the reference numeral 97, and the end point at h = 1.7 μm is indicated by the reference numeral 98. The curve course between the extreme values ​​with increasing node height is indicated by arrows, and the white point 82 and the screen triangle 84 are also drawn in the color space 86 for orientation.

[0101] Since the number of steps is the same, the step height h i There is a direct relationship between and the segment height h of the segmented mirror. Figure 8 It can be seen that, with a suitable choice of step height, the Bragg step structure according to the invention can cover a very large color space range. Therefore, true color images or true color patterns can also be easily generated by such a step structure.

[0102] Figure 8 It has further been shown that the Bragg interference structure according to the invention has a high brightness in reflection, see in particular Figure 8 (a) Brightness L*. The color impression of the structure is essentially determined by the step height h i and the refractive index n of the surrounding dielectric. Conversely, the step width w i The distance W to adjacent surface elements has only a minor influence on the resulting color impression.

[0103] Fig. 9 The influence of the size of the surface elements of the step 52 on the reflection behavior is illustrated. Fig. 9 The reflectance spectrum is for Figure 6 (b) is a Bragg step structure with the same parameters as in (b), but with a width of W = 3 μm instead of W = 6 μm. The reduction in width means that the width w of each individual metal strip is i Graph 100 shows the corresponding reduction in the step height h i =300nm (reflection spectrum 102), h i = 360nm (reflection spectrum 104) and h i =400nm (reflection spectrum 106). Fig. 9 and Figure 6 From the comparison of (b), it can be seen that the positions of the maximum values ​​of the reflection curves 102, 104, 106 or 72, 74, 76 are almost the same, but since the metal strip is narrower, the intensity of the reflection is significantly reduced.

[0104] Determination of LCh parameters from reflectance spectra 102, 104, 106 and Figure 7 The comparison of the parameters also shows that Fig. 9 and Figure 6 The hue h° and color saturation C* of the step structure of (b) are very similar to each other, and basically only Fig. 9 The brightness L* of the structure is reduced due to the smaller surface elements.

[0105] Fig.10 The effect of the number of steps on the color characteristics of the reflection is illustrated. Fig.10 (a) shows a graph 110 for different step heights h i =140nm (reflection spectrum 112), h i =175nm (reflection spectrum 114) and h i = 210 nm (reflection spectrum 116) shows the reflection spectrum of the Bragg interference structure with 9 levels of profile, and Fig.10 In the diagram 120 of (b), for h i =300nm (reflection spectrum 122), hi =360nm (reflection spectrum 124) and h i = 400nm (reflection spectrum 126) shows the reflection spectrum. Other parameters of the interference structure are Figure 6 The parameters are consistent.

[0106] Fig.10 and Figure 6 A comparison of the spectral characteristics of the different order heights shows that the spectral characteristics are almost unchanged, except that the half-value width of the spectrum is smaller at higher order numbers. Therefore, coloring by superposition of Bragg interference structures can be easily performed with different numbers of orders and thus also for segmented mirrors of different steepness.

[0107] exist Fig.11 The influence of different thickness s of the metal coating on the reflection characteristics is shown in Figure 6 In the Bragg interference structure of (a), the layer thickness of the step-level aluminum coating is s=40 nm. Based on the Bragg interference structure, the thickness of the aluminum layer is additionally calculated to be s=20 nm (e.g. Fig.11 (a)) and s = 80 nm (as shown Fig.11 (b) shows the reflectance spectrum of the structure. The remaining parameters are similar to Figure 6 The parameters of the structure of (a) are the same.

[0108] In the diagram 130, reference numerals 132, 134, 136 indicate the time for h when s=20 nm. i =140nm, h i =175nm and h i =210nm, and in the diagram 140, reference numerals 142, 144, 146 indicate the reflection spectrum for h at s=80nm. i =140nm, h i =175nm and h i =The corresponding reflection spectrum at 210 nm. Figure 6 (a) Fig.11 (a) and Fig.11 Comparison of (b) shows that the choice of the layer thickness of the aluminum layer has hardly any influence on the color characteristics.

[0109] Finally, the angular dependence of the generated hue is investigated in more detail using a four-level Bragg interference structure with an aluminum coating as an example. Fig.12 The diagram 150 shows that, on the one hand, when the light incident angle is θ=0°, for a step height h i = 300nm, 360nm and 400nm Bragg interference structures show Figure 6(b) already shows the reflection spectra 72, 74, 76. In addition, the reflection spectra 152, 154, 156 for the same step height are plotted at a light incident angle of θ = 20°. The diagram 150 clearly shows that the maximum value of the reflection spectrum hardly changes due to a change of the light incident angle of 20° or a tilt of the structure by 20°.

[0110] The only weak angular dependence of the color impression can also be Fig.13 See, its Fig.13 The CIE color diagram (a) compares the color coordinates 72', 74', 76' when the light incident angle is θ = 0° and the color coordinates 152', 154', 156' when the light incident angle is θ = 20°. In addition, Fig.13 (b) shows the lightness L* calculated in the LCh color space as curve 160 (θ=0°) or curve 162 (θ=20°), Fig.13 (c) shows the chromaticity C* as curve 170 (θ=0°) or curve 172 (θ=20°) and Fig.13 (d) shows the hue h° as curve 180 (θ=0°) or curve 182 (θ=20°).

[0111] Fig.12 and Fig.13 It is shown that the Bragg interference structure 50 according to the invention does not need to be specially adjusted to the differently inclined surface elements of the coarse structure 30 in order to produce a uniform color tone in reflection. This high angular tolerance ensures a true color reproduction even with diffuse light sources.

[0112] The angular tolerances relevant to the color impression apply when the distance between the Bragg planes does not change. This is no longer the case, at least for larger changes in the angle α between the step and the z-direction.

[0113] When manufacturing the security element according to the invention, it can be done, for example, in the following manner. The original of the stepped Bragg interference structure can be produced, for example, by a lithographic method. In a variant, a sawtooth-shaped Fresnel structure can be exposed as a coarse structure 190 using a laser writer. Here, the structure width of each individual surface element 192 is usually about 5 μm. In a subsequent step, this coarse structure is reproduced again after development, for example in an ormocer (modified ceramic) and covered with a negative photosensitive lacquer 194, such as Fig.14 (a) shown.

[0114] The desired superimposed step-shaped fine structure 196 is then exposed in precise registration with the Fresnel coarse structure 190 by an electron beam exposure device. For this purpose, a grayscale exposure method can be used, as is also used in the production of blazed gratings, because Fig.14 The order 198 illustrated in (b) corresponds to the geometry of the individual blazed gratings located on the inclined surface elements of the coarse structure. After removal of the exposed photosensitive lacquer 194, the desired superposition of the Fresnel coarse structure 190 and the Bragg fine structure 196 is formed, as shown in FIG. Fig.14 (b) as shown.

[0115] Alternatively, this composite structure can also be exposed directly in a single process step by a laser writer based on a two-photon absorption process. Another, but more complex, alternative is the grayscale exposure of the superimposed coarse and fine structures in a single exposure process by an electron beam exposure device. In another alternative, the grayscale exposure can be carried out in a two-stage exposure process, wherein the coarse structure is firstly written by an electron beam and the fine structure is written by an electron beam in a second exposure process.

[0116] Independent of the manufacturing method, the originals thus manufactured can be reproduced electroplated or by nanoimprinting. In addition, multiple arrangements of the original pattern by hot embossing or by nanoimprinting on a mold are suitable for manufacturing an impression cylinder for subsequent replication. Such an impression cylinder can realize continuous replication of the original structure in UV lacquer on a film in a roll-to-roll process. It is very advantageous here that the stepped Bragg interference structure according to the invention has a low aspect ratio. Therefore, unlike when manufacturing other known nanostructures, there is almost no adhesion of lacquer in the impression cylinder. This significantly reduces the manufacturing costs and increases the yield in manufacturing.

[0117] Finally, the embossed structures of the films are covered with a metal layer. Electron beam evaporation, thermal evaporation or spraying are preferred. Aluminum, silver, gold, nickel or chromium and alloys of these metals are particularly suitable as metals. The thickness of the metal coating is in the range of about 10 nm to about 150 nm. Alternatively, the embossed structure can also be covered with metal printing or with a metal foil.

[0118] Instead of metallization (also known as metallization), a coating made of a material with a high refractive index can also be formed on the embossed structure. As high-refractive coating materials, ZnS, ZnO, ZnSe, SiN x 、SiO x , Cr 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , Ta 2 O 5 and ZrO 2. Furthermore, semimetals such as Si or Ge can alternatively also be used as coating materials.

[0119] The metallized surface or the surface coated with a highly refractive material or semi-metallic coating is then advantageously covered with a transparent protective layer or bonded to a cover film in order to subsequently protect the surface from environmental influences.

[0120] Furthermore, it is possible to assemble the previously described elements in exact register with other element structures by means of nanoimprinting methods, for example. This method is particularly suitable for combining these Bragg interference structures with other known structures, such as relief holograms, micromirrors or nanostructures.

[0121] A multi-level etching process or a multi-level imprinting mold can also be used to manufacture a stepped structure with multiple levels.

[0122] In another variant, the regions of the embossed structure can be printed with a washable ink before the metallization process, and then (after the metallization is applied) the washable ink is washed off in a washing step together with the layer regions of the metallization applied thereon. This makes it possible to produce patterns with optically neutral or transparent regions.

[0123] List of Reference Numerals

[0124] 10 banknotes

[0125] 12 Transfer elements

[0126] 14 True color portraits created by arching

[0127] 16 security thread

[0128] 18 Arched Stars

[0129] 20 Anti-counterfeiting elements

[0130] Sub-areas 20-A, 20-B, 20-C

[0131] 22 Reflective surface area

[0132] 24 Embossed paint

[0133] 26Metallization Department

[0134] 28 embossed structure

[0135] 30 Coarse structure

[0136] 32 Segmented mirror for directional reflection

[0137] 34, 36, 38 dielectric layer

[0138] 40 3D views

[0139] 40-A, 40-B, 40-C security elements

[0140] 42 Contour Lines

[0141] 44 Gradients

[0142] 50, 50-A, 50-B produce color fine structure

[0143] 52 metallization steps

[0144] 60 Icons

[0145] 62, 64, 66 reflectance spectra

[0146] 62', 64', 66' color coordinates

[0147] 70 Icons

[0148] 72, 74, 76 reflectance spectra

[0149] 72', 74', 76' color coordinates

[0150] 80CIE Chromaticity Diagram

[0151] 82 White Spot

[0152] 84 screen triangle

[0153] 86 Color Space

[0154] 90, 92, 94, 96 curves

[0155] 97, 98 starting point and end point

[0156] 100 Icons

[0157] 102, 104, 106 reflectance spectra

[0158] 110 Icons

[0159] 112, 114, 116 reflectance spectra

[0160] 120 Icons

[0161] 122, 124, 126 reflectance spectra

[0162] 130 Icons

[0163] 132, 134, 136 reflectance spectra

[0164] 140 Icons

[0165] 142, 144, 146 reflectance spectra

[0166] 150 Icons

[0167] 152, 154, 156 reflectance spectra

[0168] 152', 154', 156' color coordinates

[0169] 160, 162 brightness curve

[0170] 170, 172 Chroma Curve

[0171] 180, 182 tone curve

[0172] 190 Rough Structure

[0173] 192 Surface Elements

[0174] 194 Photosensitive paint

[0175] 196 Fine Structure

[0176] 198 steps.

Claims

1. A security element for providing security for valuable articles, comprising a reflective surface area, the extension of which defines an xy plane and which produces a multi-colored three-dimensional image when viewed in reflected light, characterized in that: the reflective surface area comprises a coarse structure for generating a three-dimensional image and a fine structure superimposed on the coarse structure, the fine structure for generating a multi-colored appearance of the image, wherein: the coarse structure comprises a plurality of directionally reflective segmented mirrors, which are oriented in such a way that the reflective surface areas can be perceived by an observer in the form of a three-dimensional image having surfaces that are protruding and / or recessed relative to their real spatial shape, and - The fine structure is formed by a stepped Bragg interference structure with steps, on which a reflection-enhancing coating is provided, wherein the steps are superimposed on a coarse structure of directional reflective segmented mirrors, and the steps produce a color effect when observed due to Bragg interference at adjacent steps, wherein the locally different heights of the steps produce locally different colors of the three-dimensional view in the reflective surface area.

2. The security element according to claim 1, characterized in that The coarse and fine structures of the reflective surface area are embedded between transparent layers.

3. The security element according to claim 2, characterized in that The transparent layer is a transparent dielectric layer.

4. The security element according to claim 1, characterized in that The coarse-structured segmented mirror has a width of between 2 μm and 30 μm in at least one spatial direction of the xy plane.

5. The security element according to claim 1, characterized in that The coarse-structured segmented mirror has a width of between 4 μm and 10 μm in at least one spatial direction of the xy plane.

6. The security element according to claim 1, characterized in that The segmented mirrors of the coarse structure are arranged aperiodically.

7. The security element according to claim 1, characterized in that The step levels superimposed on the coarse structure have the same height within each segmented mirror.

8. The security element according to claim 1, characterized in that A Bragg interference structure having 2 to 20 steps is superimposed on the coarsely structured segmented mirror in the colored region.

9. The security element according to claim 1, characterized in that The height of the step is between 100 nm and 600 nm.

10. The security element according to claim 1, characterized in that The height of the step is between 125 nm and 520 nm.

11. The security element according to claim 1, characterized in that The height of the step is between 150 nm and 425 nm.

12. The security element according to claim 1, characterized in that In a flatter segmented mirror with a slope less than a preset threshold, the step height is less than 275 nm, and in a steeper segmented mirror with a slope greater than the threshold, the step height is greater than 275 nm.

13. The security element according to claim 1, characterized in that The steps of the metallization enclose an angle α of between 85° and 120° with the z-axis.

14. The security element according to claim 13, characterized in that The angle α is not equal to 90°.

15. The security element according to claim 1, characterized in that The reflection-increasing coating of the steps is produced by metallization.

16. The security element according to claim 15, characterized in that The metallization is selected from the metals Al, Ag, Au, Ni, Cr or an alloy of at least two of these metals.

17. The security element according to claim 15, characterized in that The thickness of the metallization is between 8 nm and 150 nm.

18. The security element according to claim 1, characterized in that The reflection-enhancing coating of the step-levels is made of a material with a higher refractive index.

19. The security element according to claim 1, characterized in that The reflection-enhancing coating of the step-levels is made of a semi-metal.

20. The security element according to claim 18, characterized in that The material with a higher refractive index is ZnS, ZnO, ZnSe, SiN x 、SiO x 、Cr2O3、Nb2O5、Ta2O5、Ti x O x Or ZrO2.

21. The security element according to claim 19, characterized in that The semi-metal is Si or Ge.

22. The security element according to claim 1, characterized in that The width of the steps is between 400 nm and 2 μm.

23. The security element according to claim 1, characterized in that The stepped Bragg interference structure produces a chromaticity C* of greater than 40 in the LCh color space in the colored region of the reflective surface region.

24. A data carrier having a security element according to any one of claims 1 to 23.

25. A data carrier according to claim 24, wherein: The data carrier is a value document.

26. The data carrier according to claim 24, wherein: The data carrier is an ID card.

27. A method for producing a security element according to any one of claims 1 to 23, in which method: - generating a reflective surface area having a coarse structure for generating a three-dimensional image and a fine structure superimposed on the coarse structure, the fine structure for generating a multi-colored appearance of the image, - the coarse structure is designed as a segmented mirror with a plurality of directional reflections, the segmented mirrors being oriented in such a way that the reflective surface areas can be perceived by an observer in the form of a three-dimensional image with a surface that is protruding and / or recessed relative to its real spatial shape, and The fine structure is formed by a stepped Bragg interference structure with steps which are superimposed on a directionally reflecting segmented mirror of the coarse structure and which, when observed, produces a color effect due to Bragg interference at adjacent steps, wherein: The steps are designed locally with different heights in order to generate locally different colors of the three-dimensional image in the reflective surface area, and a reflection-enhancing coating is provided on the steps.

Images