WO2021052630A1

WO2021052630A1 - Method for producing a security element, and security element

Info

Publication number: WO2021052630A1
Application number: PCT/EP2020/025418
Authority: WO
Inventors: Tobias Sattler; Thomas Gerhardt; Michael Rahm; Christian Fuhse; Katalin Szendrei-Temesi; Kerstin GOTTSCHLING; Udo SCHAUMBURGER; Maik Rudolf Johann Scherer
Original assignee: Giesecke+Devrient Currency Technology Gmbh
Priority date: 2019-09-20
Filing date: 2020-09-16
Publication date: 2021-03-25
Also published as: EP4031380B1; EP4031380A1; CN114423619B; CN114423619A

Abstract

The invention relates to a method for producing a security element (4) for producing value documents, such as banknotes (2), checks, or the like, wherein the production method has the following steps: providing a polymer film (12), forming a laterally structured microfibril structure (13) in the polymer film (12) under the effect of standing light waves (16, 16, 16b) using the interference between a reference radiation and a modulated object radiation in order to produce location-dependent crosslinking in the polymer film (12), and developing the film using a solvent such that the polymer film (12) produces a colorful optically variable motif in plan view of the security element (4).

Description

Manufacturing process for a

Security element and security element The invention relates to a production method for a security element that generates a colorful, optically variable motif. The invention further relates to such a security element.

Photosensitive layers which change their physical and chemical properties when they are exposed to irradiation by light are known in the prior art. They are mainly used for the production of micro and nanostructures. The structures are created with the help of UV lacquers or so-called photoresists, i.e. ultimately with the methods of microlithography.

One way of producing layers with iridescent colors are so-called color shift layer structures. Although they can be applied over the entire surface of a substrate using a vapor deposition process, multicolored structures require many operations and are therefore complex to manufacture. In particular, registration problems must be taken into account in individual work steps.

Volume holograms are also known, which produce a three-dimensional image in the viewer through light diffraction and interference. Volume holograms store both the intensity and the phases of incident light rays in a light-sensitive medium. The large-scale production of volume holograms is complicated and expensive, since complex equipment is required for exposure and, as a rule, expensive photoresists have to be used for production. From the publication M. Ito et al., "Structural color using organized microfibrillation in glassy polymer films", Nature, June 20, 2019, Vol. 570, pages 363-367, it is known to add a polymer by standing light waves network and thus to form a structure from microfibrils that create a colorful motif.

The invention is based on the object of specifying a production method for the simple production of a security element that generates a colored, optically variable motif, as well as such a security element.

The invention is defined in the independent claims; the dependent claims relate to preferred developments.

The optically variable security element for producing documents of value, such as bank notes, checks or the like, has a polymer film and optionally a reflector layer arranged under the polymer film. Laterally structured microfibrils are formed in the polymer film, which give the polymer film a color effect that appears as a colorful motif. The microfibrils are produced according to the principle described in the Nature article mentioned. They are therefore organized according to standing light waves. These originate, for example, from the interference of coherent light rays, which lead to places of constructive interference in the polymer film and thus create a (usually locally varying) cross-linking in the polymer, which is then exposed using suitable solvents.

This microfibrillation process described in Nature is carried out in a variant 1 with a reflector layer that is structured laterally with regard to the degree of reflection and / or profiling, so that when loading In the incident light above the reflector layer, the interference between falling and back-reflected radiation and so ultimately the microfibril structure for the colorful motif is created. The reflector layer ensures the networking in such a way that the microfibrils have the lateral structure that leads to the colorful motif. The reflector layer can, for. B. be designed as a metallic mirror layer and are also removed after the microfibrillation process. In a variant 2, interference between two incident beams is generated in the polymer film. Then you do not need a reflector layer and can, for example, write a volume hologram directly into the polymer film.

The polymer film is thus designed by microfibrillation with regard to a color effect, in particular as a volume hologram, and has a sponge-like structure in which individual planes and / or holes are arranged periodically. This creates structure colors.

The polymer film is quasi "exposed" by light that is applied in the form of standing waves, whereby in variant 1 the lateral structuring of the reflector layer also structures the exposure, and is subsequently developed by solvent-based removal of the non-crosslinked components structured reflector layer for the exposure step, the motif is very easy to produce.

The use of water-soluble polymers such as PVA, PA, PVP, PAA or PAM is particularly advantageous and environmentally friendly. With these, the non-crosslinked areas can only be removed with water and thus without solvents. In embodiments, the reflector layer is modulated laterally, in particular pixelated. This can be done, for example, in the form of a pixel structure. Additionally / alternatively, the distance between the reflector layer and the polymer layer can also be modulated laterally. The degree of reflection can be varied, for example, via the thickness of a metallic layer, which can fluctuate between a maximum reflective layer thickness and zero. Metals such as Al, Cu, Cr, Ni, Au, Ag etc. and their alloys are preferred as materials for the reflector layer, as well as ZnS, SiO2, MgF2 or thin-film interference layer systems known from the prior art. Special dielectrics or layer systems made from the materials mentioned are also possible.

When microfibrillation is formed, the reflector layer generates an intensity modulation of a depicted motif in the polymer, which in turn leads to the formation of laterally structured microfibrils during the development process. These can be viewed as laterally structured Bragges that cause a color component. In particular, a laterally varying structuring of the polymer film can then be dispensed with. Since a reflector layer can be produced relatively easily and structured laterally, these embodiments have particular advantages with regard to ease of production. However, they are difficult to imitate or even fake.

Of course, as an alternative or as an additional development, it is equally possible to structure the polymer film laterally with regard to the color effect. This can e.g. B. can be achieved by appropriate exposure to standing light waves that differ laterally, spectrally and / or in intensity. In embodiments, the microfibril structure provides a type of volume hologram. For this purpose, the polymer film is exposed in embodiments, as is known for reflection, transmission or Denisjuk holograms. In other embodiments, the reflector layer is designed as a relief structure which has a corresponding relief to represent a motif, so that during exposure the light reflected from this laterally structured surface in the sense of an object beam with the incident beam, which serves as a reference, in the polymer layer to overlay and thus interference occurs. The surface relief ensures phase modulation of the light reflected by the reflective layer, so that this relief structure has an effect on the color effect.

The relief structure can in particular bring about a laterally varying distance between the polymer layer and reflector layer. It can be produced with the methods of micro- and nanolithography and transferred or multiplied by various embossing processes. In particular, the relief structure can have: plateaus in different flea levels, which influence the color effect, at different distances from the polymer structure, micromirrors, a blazed grating structure, a Fresnel structure, a moth's eye structure, a subwavelength grating with sharp or rounded edges, a sinusoidal grating structure, a columnar structure or a Step grating structure with sloping or vertical flanks. Various of these grid structures can also be used in adjacent areas. The structures mentioned can also overlap, especially if the mean periods or quasi-periods or the individual structure sizes have different orders of magnitude. In general, the relief structure can be designed as a free-form surface. For example, motifs typical of banknotes can be displayed, with three-dimensional motifs also being possible. The optically variable effect is at suitable lighting can also be observed without a laterally structured reflective layer.

In further embodiments, the microfibril structure generates a multicolored pixel image, with a pixel structure which brings about the pixel image being formed in the reflector layer (if not removed), the polymer layer or in the case of the.

In a further embodiment, the microfibril structure is structured perpendicular to the surface of the polymer film. The sponge-like structure of the polymer film, in which the individual layers and / or the holes are arranged, is produced in such a way that the standing waves with which the polymer film is exposed are applied in such a way that the microfibril structure does not form periodically when the Polymer film comes into contact with the solvent. This can be achieved, for example, by irradiating the polymer layer with standing waves of different wavelengths.

In a first variant of this embodiment, sturgeon sites are specifically built into the microfibril structure. For example, individual layers in the microfibril structure that are made thicker or thinner than the other layers of the microfibril structure through the irradiation with the standing waves and the subsequent solvent-based removal of non-crosslinked components in the polymer film are designated as defects. This creates a minimum / maximum in the reflection spectrum within the highly reflective area (band gap) and the color impression changes compared to a periodic formation of the microfibril structure. In a second variant of this embodiment, the thickness of the layers of the microfibril structure is formed by irradiating the polymer structure with the standing waves in such a way that the thickness of the layers increases continuously or gradually the further away the layers are from the substrate. This creates a broadening of the reflection spectrum. The increase in the thickness of the layers can be achieved, for example, by using different photoinitiators, the further the layers are away from the substrate. Photoinitiators are chemical compounds that break down after they have absorbed light, thus forming reactive particles that can start a reaction. The photoinitiators are exposed to light sources of different wavelengths and this results in different layer thicknesses in the different layers, e.g. one photoinitiator A for the layers that are closer to the substrate, another photoinitiator B and for the layers that are further away from the substrate another photoinitiator C can be used for the layers that are furthest away from the substrate. Photoinitiators are chemical compounds that break down after they have absorbed light, thus forming reactive particles that can start a reaction. The photo initiators are exposed to light sources of different wavelengths and this results in different layer thicknesses in the different layers.

In modifications, different polymer materials are used for the individual polymer layers and the same photoinitiator. Different polymers produce different band gaps because the layer thicknesses and the refractive indices vary depending on the polymer. In execution forms, several polymer films of different polymers are applied in several operations, and the exposure and development are carried out in one operation. Development here means the solvent-based removal of the non-crosslinked constituents of the polymers. Different developer mixtures, such as concentrations of acetic acid in water or mixtures of acetic acid in different solvents, develop the different layers of the microfibril structure in the polymer film at different speeds. In modifications, the described layer thickness variation is achieved in that the development time, i.e. the time required by individual layers for the solvent-based removal of the non-crosslinked components of the polymers, is set differently for the layers so that, for example, upper layers are already fully developed and lower not yet, or only partially. One type of polymer and photoinitiator can be used throughout, and exposure and development are carried out in one operation.

This use of different developer mixtures can be used not only for structuring the microfibril structure perpendicular to the surface of the polymer film, but also for lateral structuring. Different developer mixtures can be used laterally next to one another on the polymer film in order to achieve different degrees of development laterally and thus different structural colors. In embodiments, the developer mixtures are applied laterally to the polymer film with a time offset. This can be done in two work steps, for example. First, a developer mix is applied to a first location on the polymer film. Then the same developer mixture is also applied to a second point on the polymer film. At the At the first point on the polymer film, the developer mixture therefore has more time to interact with the polymer film than at the second point, and development has progressed further at the first point, as a result of which different structural colors are generated at laterally different points.

According to a further embodiment, the polymer layer is designed to be elastically or plastically deformable. As a result, a reversible or irreversible change in the color of the polymer layer can be produced by mechanical pressure on part or the entire surface of the polymer layer by changing the layer thickness of the polymer layer. The color or the reflected (or transmitted) wavelength of the polymer layer is changed by changing its layer thickness; the thinner the layer, the more the color impression shifts to bluish. If the polymer layer is red, for example, and mechanical pressure is exerted on it with a finger, the polymer layer is compressed in this area, the layer thickness becomes smaller and the structure color can change from red to yellow and green to blue, for example .

A plastic and thus permanent deformation of the polymer layer takes place, for example, using an embossing process, preferably using intaglio printing.

The sponge-like structure of the polymer film consists of microfibrils and cavities. If these cavities are at least partially open-pored, they can, according to a further embodiment, be filled with a liquid or gaseous medium. If, for example, part of the cavities is filled with water and air remains in another part, regions in the area of the filled cavities will have different colors, because air has a refractive index of 1 and water of 1.33. It is also possible for the polymer layer to become transparent when the refractive index of the microfibrils and of the medium in the cavities are the same. According to a further preferred embodiment, the reflector layer can remain on the polymer layer. Alternatively, the polymer layer can be coated with a dark or black color on the surface facing away from viewing. Alternatively, the polymer layer can be modified by adding carbon black or carbon black in order to bring out the best of the interference colors. To further increase the color intensity, transparent high-index particles, for example made from 1102, are added to the polymer so that the difference in refractive index between the fibrillated layers and continuous layers is increased further. Suppression with a diffusely scattering color also leads to different, in particular complementary colors in the case of a specular or non-specular view.

The aspects mentioned in relation to the security element naturally also apply equally to the production process which, with regard to the production of the microfibrils, follows the principle mentioned in the above-mentioned Nature article.

In particular, a security element is provided that is manufactured or obtainable using one of the manufacturing processes mentioned.

According to a further preferred embodiment, the polymer layer according to the invention is cured in a subsequent process step.

This has the particular advantage that the resistance of the polymer layer to mechanical abrasion, subsequent mechanical pressure, solvents or other environmental influences, for example, is increased. The hardening takes place particularly preferably by irradiating the polymer layer with ultraviolet radiation. A further increase in the resistance of the polymer layer and of the reflector layer remaining on the polymer layer results from embedding the polymer layer between protective layers and / or the foils.

The invention also relates to a value document with a security element of the type mentioned. In one embodiment, the value document is designed, for example, as a bank note or check.

The security element according to the invention can be combined with any other security elements of the document of value, for example holograms, micromirrors, e.g. with running effects or 3D surfaces (Fresnel-like), micro-concave mirrors or subwavelength structures. This is preferably done in each case in such a way that the reflective layer generates the reflection for generating the standing wave and generates the other features described in other lateral subregions. Optionally, the part that generates the standing wave is removed after exposure, which can be done, for example, by etching. It can also be combined with the following features: magnet, conductivity, fluorescence, phosphorescence. These arbitrary other security elements are particularly preferably arranged laterally next to the microfibrils.

The invention is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings, which also reveal features essential to the invention. These exemplary embodiments are used for illustration purposes only and are not to be construed as restrictive. For example, a description of an execution For example, with a large number of elements or components, it should not be interpreted in such a way that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments can also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments can be combined with one another, unless otherwise stated. Modifications and variations which are described for one of the exemplary embodiments can also be applied to other exemplary embodiments. To avoid repetition, the same or mutually corresponding elements in different figures are denoted by the same reference numerals and are not explained more than once. In the figures show:

1 shows a schematic representation of a bank note with several security elements,

2 shows a sectional illustration through one of the security elements of FIG. 1, FIG. 3 shows a schematic sectional illustration of a polymer layer in the security element of FIG. 2,

3A to 3D different possibilities for exposure of the polymer layer in order to form the structure according to FIG. 3,

4 shows a security element similar to that of FIG. 2, but with a reflector layer structured in a pixel-like manner, FIG. 5 shows a sectional illustration similar to FIG. 2, but with a polymer layer structured in a pixel-like manner,

6 shows a representation similar to FIGS. 4 and 5, with both the reflector layer and the polymer layer being structured in a pixel-like manner,

7 shows an embodiment of the security element with a reflector layer which has different plateaus,

8 shows an illustration of an embodiment similar to FIG. 2 for providing a security feature in the form of a volume hologram; and FIG. 9 shows an illustration of an embodiment for providing a security feature in the form of a volume hologram, two interfering beams being used in the clearance position. Fig. 1 shows schematically a plan view of a banknote 2, which has several security elements. One security element 4 is designed in the form of a patch, another security element in the form of a security strip or security thread 6. The specific two-dimensional design of the security element can be selected as a function of the application. The description below relates purely by way of example to the security element 4.

Fig. 2 shows a sectional view through the security element 4. It is placed on a substrate 8, for example banknote paper of the banknote 2, an intermediate carrier also being used as the substrate 8 can, which is then applied to a banknote paper of the banknote 2, so that the security element 4 is then formed as a so-called transfer element. On the substrate 8 there is a polymer layer 12 which has been provided with a microfibril structure 13 from its upper side 14 using the process described in the above-mentioned Nature article. This is shown schematically in FIG. 3 and organized in accordance with standing waves that arose from interference between an object and a reference beam. There are two variants for providing these rays: Variant 1 works with a reflector layer under the polymer layer. Then an incident beam is the reference beam, the beam reflected at the reflector layer is the object beam. In an alternative variant 2, the object and reference beams are irradiated independently.

The microfibril structure 13 comprises microfibrils 13a and cavities 13b (cf. FIG. 3). In variant 1, it is generated by irradiating a commercially available, flat polymer such as a polystyrene or polycarbonate film or a corresponding film (preferably parallel to its surface normal) with radiation whose coherence length is greater than the thickness of the polymer layer 12 is. UV radiation is preferably used. During this irradiation there is a reflector layer under the polymer layer 12 (cf. FIGS. 3-8), so that the object beam is created in back reflection. In variant 2 (Fig. 9), the object and reference beams are irradiated independently. In both cases, the thickness of the polymer layer 12 is less than the coherence length of the radiation (s) used, as a result of which standing waves are formed within the polymer layer 12. At wave peaks where the intensity of the standing waves is maximum, the po- lymer networked, and a periodic mechanical stress field is formed between networked and non-networked areas, the latter being at nodes of the standing waves. The polymer is preferably mixed with an additional photoinitiator.

By using a suitable solvent, the microfibril structure 13 exposed in this way is then formed according to FIG. 3, individual levels of the micro-eyeglasses 13a and the cavities 13b being automatically arranged periodically according to the standing wave structure. With regard to the details of the production, reference is made to the Nature article including the associated supplementary material published in Nature. These publications are hereby fully integrated in terms of content.

The polymer layer 12, when exposed and developed in this way, produces a laterally modulated color effect which, in variant 1, is influenced by a lateral structuring of the reflector layer 10 underneath during exposure - even if the polymer layer 12 in the security element has no underlying Reflector layer 10 is used.

Structuring can take place during the exposure, ie the microfibril structure 13 consisting of microfibrils 13 a and cavities 13 b can be structured laterally, that is to say transversely to the surface 14. 3A shows an embodiment in which a wide-field exposure 16 exposes the entire polymer layer 12 uniformly. An additional laterally structured reflector layer 10 creates a laterally structured microfibril structure 13 and thus the colorful motif. The substrate 8 on which the polymer layer 12 is located is not shown here and below. It can be arranged between the polymer layer 12 and the reflector layer 10 or on the upper side or the side of the polymer layer 12 facing the exposure 16. In both cases, the substrate 10 must be transparent to the illumination wavelengths required for structuring the polymer layer 12.

FIG. 3B shows that by using a mask 18 a structured exposure can also take place. The mask 18 blocks the wide-field exposure 16 at individual points, so that light can only incident on the polymer layer 12 at the gaps in the mask 18. Accordingly, light can only be reflected on the reflector layer in these areas and interfere with the incident light, provided that there is a reflection effect in these illuminated areas. The color effect is only created at the points where light falls through the mask and is also reflected at the same time on the reflector layer. In this way, it is particularly possible, please include, in addition or as an alternative to the structuring that is effected by the reflector layer 10, a structuring, for. B. to form pixelation in the polymer layer 12, this pixelation having an effect on the color. The exposure can also take place with different wavelengths, so that the color tone generated by the polymer layer 12 laterally under different, z. B. with three-colored pixels, which are formed from sub-pixels in primary colors (z. B. red, green, blue) can be. For this purpose, several wide-field exposures 16a, 16b at different wavelengths or in different wavelength ranges are used one after the other. The additional lateral structuring takes place by means of different masks 18a, 18b, which each act on the corresponding wide-field exposure 16a, 16b. 3C illustrates these exposures taking place sequentially one after the other in a common representation. The multicolor is not restricted to two colors, of course; similarly, three, four or more different exposure steps can also take place, each exposure step exposing a different partial surface area of the polymer layer 12 and providing it with a color effect. When exposed several times in this way, the polymer layer 12 is developed by using the solvent in order to form the laterally structured microfibril structure 13.

According to a preferred embodiment, the mask remains on the security feature, for example in order to form holograms or other optically variable features in certain areas. A metal layer existing in certain areas is preferably used as a mask for this purpose. The mask is preferably removed from the polymer layer in areas after exposure. The mask remaining on the substrate is particularly preferably transparent in the visible spectral range (ie not visible or at least inconspicuous in the end product) and opaque or at least semitransparent in the UV range. Here, the mask consists, for example, of a 50 nm thick layer of T1O2. This is largely transparent in the visible range, but shows only a very low transmission in the UV range at wavelengths around or below 300 nm.

Alternatively, the mask can be separated from the film web. It should be noted that in the finished security element 4, 6, the tilt angle-dependent color effect is also created by interference when illuminated - possibly even without a reflector layer 10. The microfibrils are created during the development of the polymer layer, with their spacing from one another can deviate from the original distance between the corrugations when imprinted due to the development process. In this way it is possible for exposure wavelengths in the UV range after the development of the polymer layer to have the Bragg maxima during observation in the visible range of the spectrum.

An alternative to wide-field exposure is exposure with a scanned light beam 20 (for example from a laser or an LED), which has the required coherence length and is deflected over the polymer layer 12 according to a scan pattern 22. This is shown in FIG. 3D. The wavelength of the laser radiation can be designed differently at the individual locations in order to generate a lateral structuring of the polymer layer 12 with regard to the color effect. These additional structuring options allow e.g. B. to provide the reflector layer 10 with a pixel structure for the motif, which relates to the brightness, and to provide each pixel with its color-setting subpixels through the additional structuring (masks or raster).

The laterally structured reflector layer 10 below the polymer layer 12 can be present over the entire area, as shown in FIGS. 2 and 3, and also over part of the area. 4 shows a pixelation of the reflector layer 10 consisting of reflector pixels 24 with high reflection and reflector pixels 26 with low reflection or without reflection. The standing wave then only forms at the pixels at which the reflector layer 10 has sufficient reflection, or the intensity depends on the degree of reflection and / or the pixel area coverage. In this way, a colored, rasterized pixel image can be generated. As already explained with reference to FIGS. 3B, 3C and 3D, the exposure can also result in additional rastering of the polymer layer 12, so that this polymer layer has pixels 28 and 30, as FIG Developing with the solvent the color impression differs. This makes a colorful pixel image possible. Of course, more than two different types of pixels are also possible.

The principle of FIGS. 4 and 5 can of course also be combined, as FIG. 6 shows. The pixel grid of reflector pixels and polymer layer pixels does not necessarily have to be identical, even if this can be an advantage. In particular, through a very high pixel density in the reflector layer, the brightness of an individual color within a color pixel which is formed by a polymer layer pixel can be set as a function of location. Ultimately, the brightness for each color point for creating a motif can thus be freely selected.

It is therefore provided in special embodiments that both the polymer layer 12 and the reflector layer 10 have a pixel structure, the pixel density in the reflector layer 10 being at least twice the pixel density of the polymer layer 12. The fact that the reflector layer 10 is responsible for the intensity at one point and the polymer layer 12 is responsible for the color can thus be used particularly favorably.

The mentioned effects are embodied in the microfibril structure and remain ben after development even without the reflector layer 10. Fig. 7 shows an embodiment in which the reflector layer 10 has plateaus 32, 34, 36 at different heights. This makes use of the fact that the path length of the reflected radiation is relevant for the placement of the nodes and wave antinodes of the standing waves within the polymer layer. Due to the different height levels, i.e. from the distances of the plateaus 32, 34, 36 to the polymer layer 12, each plateau 32, 34, 36 generates a different color intensity with an otherwise unchanged polymer layer 12. This approach comes in particular with a non-structured polymer layer 12, like them is obtained, for example, with the wide-field exposure 16 according to FIG. 3A, in question, in order to distribute different color intensities in a laterally structured manner.

8 shows an embodiment in which the reflector layer 10 has a relief structure on its side facing the polymer layer 12. In this way, as already explained in the general part of the description, a security element similar to a volume hologram can be generated in a particularly simple manner in that the relief structure is designed in such a way that it reflects, for example, a three-dimensional optical impression. The exposure with this reflector layer 10 then creates a corresponding object beam and a total of a polymer containing microfibrils, the microfibrils functioning similarly to the Braggens in a volume hologram that offers a three-dimensional display from a wide variety of angles. In the manufacturing method in variant 2, according to FIG. 9, the object and reference beams are radiated as separate beams 42, 44 capable of interference. The object beam 44 does not originate from the back reflection of the reference beam, as was the case with variant 1. It is therefore not a reflector Shift provided. Rather, the object beam 44 is modulated as in the case of a holographic recording of an object 46. Alternatively, the modulation is generated by means of optical beam shaping elements (eg DMD or similar). The two (or more) beams can be irradiated from the same or from opposite sides of the polymer layer 12.

In particular, the following designs and configurations are possible:

1. Light sources a. Full / partial exposure:

The light source can expose the polymer layer in the form of a polymer film over its entire area and over part of its area. For a full-area exposure z. B. LEDs sufficient (Fig. 3A), whose coherence length is sufficient. If the film is to be exposed in pixels or areas, a deflectable, strongly focused light beam (e.g. laser) can be used (Fig. 3D), or light sources that can be directly modulated (micro-LED) or optical elements such as SLM ( spatial light modulator), DMD (digital micromirror device) or DOE (diffractive optical element). Another way to produce a partial exposure Be is to use a mask (Fig. 3B, 3C).

A full or at least partial area lighting can also be provided by a self-illuminating display or a self-illuminating screen. The display or the screen illuminates the entire surface or the polymer in a pattern.

In the case of an industrial manufacturing process based on the roll-to-roll (R2R) principle, the exposure can alternatively also be arranged in lines. Nete LEDs take place, the line being aligned parallel to the axis of rotation of a roller. The polymer is illuminated directly by the LEDs or by imaging optics between the LEDs and the polymer. In all cases, a coherent superposition of an object beam and a reference beam in the polymer is required.

The microfibrillation process has the advantage that by using DOE / SLM / DMD in combination with LEDs or lasers as a light source, light can be modulated on micrometer length scales. Resolutions of up to 25,000 DPI can thus be achieved with the microfibrillation process. At the same time, the flexibility of the optical elements enables a high degree of customization. Since the microfibrils that represent the Bragges are embedded in the polymer film, no impressions or impressions can be made for counterfeiting purposes, which leads to a high degree of protection against forgery. b. Variation of the wavelength:

Irradiation with light of different wavelengths produces different structural colors. Thus, by additive color mixing of RGB pixels, it is possible to cover a large color space. The exposures could be generated one after the other or simultaneously by means of a differently colored display on the display, monochromatic lasers or LEDs with different emission wavelengths. For this purpose, the polymer film could, for example, be covered with different masks 18 one after the other and exposed to monochromatic radiation through them (FIG. 3D).

2. Area coverage of the reflective layer The reflective layer underneath the polymer can be present over the entire area or over part of the area. If the reflective layer is over the entire surface, a pixel-by-pixel rasterization of the color could only take place via the modulation of the light source. If the reflective layer is rasterized, standing waves are only formed in the pixels below which a reflective layer is present. It is thus possible to create a raster.

3. Relief structure of the reflective layer

It is possible to place an embossed, reflective relief structure under the polymer film. The embossed structure can consist of all possible relief structures such as micromirrors, Fresnel-like micromirrors, blazed gratings, moth-eye structures, subwavelength gratings, sinusoidal gratings, man-hattan gratings or Aztec structures. Structuring by applying a dark color, for example, is also possible, the color being applied to the side of the relief structure that faces the lighting. These and other structures can also be arranged next to one another or overlay one another. For example, it is possible to use moth-eye structures or other light-absorbing structures to suppress the reflection in certain areas and to not generate any microfibrils in the polymer layer in these areas.

The relief master can either a) be located directly on the foil and remain there, b) be located directly on the foil and, after exposure, be peeled off in a transfer step or at least partially removed (for

Example by completely or partially etching away the metallic coating, while the relief structure remains), c) run along in register below the film, d) are stationary under the film (do not run along), the exposure takes place in the register z. B. by a synchronized flash light source.

The following embodiments are preferred:

I. Single-color pixel image with structure colors:

Using one of the in la. and 2. techniques described, a monochrome, iridescent pixel image can be produced. By varying the area coverage of colored areas, different saturations of the hue can also be produced. The motifs produced have a resolution of up to 25,000 DPI.

Because of the high resolution, the use of the pixel image is also advantageous for security features with micro-imaging elements such as microlenses, wherein the pixel image can function as a micro-structure image in the focal plane of the micro-imaging elements.

II. Multi-colored pixel image with structure colors

Using a combination of that in la. and 2. techniques described as well as in lb. Using light sources with different wavelengths described, a multicolored, iridescent pixel image can be produced. By varying the area coverage of colored areas, different saturations of the hue can also be produced. The motifs produced have a resolution of up to 25,000 DPI. Because of the high resolution, the use of the pixel image is also sensible for security features with micro-imaging elements such as micro-lenses. This would make it possible to upgrade existing microlens features in the bank note market, as these have only been single-colored to date.

III. Production of microfibrillation holograms Microfibrillation holograms are volume holograms that are produced using the microfibrillation process. With the microfibrillation process, a photoresist can be replaced by the polymer film, e.g. commercially available polymers mixed with small amounts of photoinitiators. The production process is otherwise identical to the usual production of volume holograms by exposure to interfering rays. Thus, the hologram can be recorded in all of the forms already described, such as. B. as a reflection, transmission or Denisjuk hologram.

Alternatively, the hologram can also be produced without using a material object. The object beam can be generated by using an SLM or DMD (reflection hologram only).

List of reference symbols

2 banknote

4 security element

6 Security thread

8 substrate

10 reflector layer

12 polymer layer

13 microfibril structure 13a microfibril 13b cavity

14 top

16, 16a, 16b wide field exposure 18, 18a, 18b mask 20 laser 22 scan pattern

24, 26 reflector pixels 28, 30 polymer layer pixels 32, 34, 36 plateau 38 embedding medium 40 relief layer 42 reference radiation 44 object radiation 46 object

Claims

Patent claims

1. Production method for a security element (4) for producing documents of value, such as bank notes (2), checks or the like, the production method having the following steps

Providing a polymer film (12) and

Forming a laterally structured microfibril structure (13) in the polymer film (12) by the action of standing light waves (16, 16, 16b) by interference of a reference radiation with a modulated object radiation to a location-dependent crosslinking in the polymer film (12) and developing with a solvent so that the polymer film (12) produces a colorful, optically variable motif when viewed from above on the security element (4).

2. The manufacturing method according to claim 1, further comprising the steps of:

Generating a reflector layer (10) lying under the polymer film (12) which has a lateral structuring with regard to degree of reflection and / or profiling, and

Radiation with a coherence length greater than a Po lymerfilmdicke on the polymer film to form the laterally structured microfibril structure (13), the reference radiation being formed by the incident radiation and the object radiation by the radiation reflected on the reflector layer.

3. The manufacturing method according to claim 2, wherein the lateral structuring of the reflector layer (10) is designed as a pixel structure which effects the pixel image.

4. Manufacturing method according to claim 2 or 3, wherein the reflector layer (10) is provided laterally modulated in such a way that it has sections with different degrees of reflection.

5. Manufacturing method according to claim 2 or 3, wherein the microfibril structure (13) is structured laterally with regard to a color generated by it, in particular in the form of a pixel structure.

6. Manufacturing method according to one of claims 2 to 5, wherein the reflector layer (10) for lateral structuring with regard to profiling is formed with a relief structure.

7. Manufacturing method according to claim 6, wherein the relief structure has at least one of the following structures: - plateaus (32, 34, 36) in different, height levels,

Micromirror, blazed lattice structure,

Fresnel structure,

Sinus grid structure, - columns of different heights,

Step grilles with straight or sloping sides, moth-eye structures and

Subwavelength structures with sharp or rounded edges.

8. The manufacturing method according to any one of claims 1 to 7, wherein the

Polymer film (12) is irradiated in such a way that laterally structured microfibril structure (13) provides a multicolored image, preferably a pixel image.

9. Manufacturing method according to one of claims 1 to 8, wherein the

Reflector layer (10) is removed after the formation of the laterally structured microfibril structure (13).

10. The manufacturing method according to any one of claims 1 to 9, wherein the

Polymer film (12) for forming the laterally structured microfibril structure (13) is irradiated with two mutually capable of interference, one of which forms the reference beam and the other is modulated and forms the object beam, so that the laterally structured microfibril structure (13) provides a volume hologram.

11. Production method according to one of claims 1 to 10, wherein the microfibril structure (13) is structured in the polymer film (10) perpendicular to the surface of the polymer film (10) with regard to layer thicknesses of individual layers of the microfibril structure (13).

12. Security element for securing documents of value, such as bank notes (2), checks or the like, with a structured microfibril structure (13) generated by a method according to one of the above claims.

13. The security element according to claim 12, wherein the laterally structured microfibril structure (13) provides a volume hologram.