EP2225110B1 - Security element - Google Patents

Description

The invention relates to a security element for security papers, documents of value and the like with a feature area which selectively influences incident electromagnetic radiation. The invention also relates to a method for producing such a security element as well as a security paper and a data carrier with such a security element.

For some years now, holograms, holographic grating images and other hologram-like diffraction structures have been used to ensure the authenticity of credit cards, bank notes and other documents of value. Metallized embossed holograms, which preferably consist of sinusoidal surface profiles with grating periods between approximately 600 nm and 2 μm, are used today on countless banknotes as a sign of their authenticity.

In order to further increase the attractiveness and protection against forgery, a large number of optically variable effects have been developed: As soon as the bank note is moved relative to the viewer and / or the light source, the hologram changes its appearance drastically. Color changes that manifest themselves in so-called run, tilt or morph effects are particularly typical. This optical variability and the metallic sheen of the metallized hologram foils ensure that real banknotes are clearly different from counterfeits that were created with the help of color printers. Comparable optical variability cannot be achieved with commercially available colors. Diffraction gratings, the basic building blocks of such holograms, basically create a spectral color split.

Despite the high level of development that the holograms used to protect banknotes against counterfeiting have meanwhile reached, better and better counterfeits are coming onto the market. The grating periods of at least 600 nm used in the holograms can be produced not only with electron beam lithography systems, but also by interferometric direct exposure using a laser, which significantly reduces the forgery-proofness of the holograms. Falsified holograms are often produced with the help of dot matrix systems, the functionality of which is ultimately also based on the interference of laser beams.

So-called moiré magnification arrangements have also been used as security features for some time. The basic mode of operation of such moiré magnification arrangements is described in the article " The moire magnifier ", MC Hutley, R. Hunt, RF Stevens and P. Savander, Pure Appl. Opt. 3 (1994), pp. 133-142 described. In short, moiré magnification denotes a phenomenon which occurs when a grid of image objects is viewed through a lenticular screen with approximately the same pitch. As with every pair of similar grids, a moiré pattern results, in which case each of the moiré strips appears in the form of an enlarged and rotated image of the elements of the image grid.

Due to the small line thickness of the letters and symbols used in such moiré magnification arrangements of approximately one micrometer, it has not previously been possible to produce colored letters through finely structured metallic surfaces. Diffraction effects are hardly an option for the coloring because lattices with the usual periods are not or only in special cases in the lines that make up the letters or symbols of the microstructure array. In addition, the lens array used for viewing equalizes the angular splitting of individual spectral colors, so that classic lattice diffraction in the first order of diffraction is not very suitable for coloring in moiré magnification arrangements or in the more general modulo magnification arrangements.

The WO 2007/140484 A describes a colored, reflective security feature that optionally has color-shifting properties. The feature contains nanoparticles and a dye that preferably modifies a light spectrum that is reflected by a reflective layer formed from the nanoparticles.

The WO 2004/077468 A2 describes a security element with an incident electromagnetic radiation selectively influencing feature area which contains metallic nanostructures in which volume or surface plasmons are excited and / or resonance phenomena are excited by the incident electromagnetic radiation and the feature area as metallic nanostructures sub-wavelength grating with grating periods below the wavelength of visible light contains. The grids form a monogrid.

On this basis, the invention is based on the object of avoiding the disadvantages of the prior art and, in particular, of creating a security element with an attractive visual appearance and high level of protection against forgery.

This object is achieved by the security element with the features of the main claim. A method for producing such a security element, a security paper and a data carrier are specified in the independent claims. Further developments of the invention are the subject of the subclaims.

According to the invention, a generic security element provides that the feature area contains metallic nanostructures in which volume or surface plasmons are excited and / or resonance phenomena are caused by the incident electromagnetic radiation.

Plasmons are collective oscillations of free electrons relative to the ion cores in metals. At the so-called plasma frequency, there is an increased absorption of the stimulating light. By recombining plasmons in radiation, light scattering can occur, especially when the metal is in particulate form. Surface plasmon polaritons (SPs) are electromagnetic radiation bound to metallic interfaces, which propagates along their boundary layer and is absorbed in the process. The excitation of surface plasmon polaritons takes place via the pulse adaptation of the incident light and the surface plasmon polaritons via a dielectric or via the reciprocal lattice vector of the periodic structuring of the metal surface.

Furthermore, unusual intensity changes in the transmission or in the reflection can occur at subwavelength gratings if the incident light leads to resonances in the interstices or in the cavities of the grating structure. Such resonance effects can also be caused by the excitation of surface plasmons or surface polaritons can be explained by the incident radiation. In the case of transmission gratings, a strong redistribution of intensity between reflection and transmission can be observed for certain wavelength ranges. These so-called cavity resonances also lead to increased absorption of light. It is worth mentioning that this effect can also cause an extraordinary increase in transmission.

Even if the mentioned physical effects are currently regarded as the correct description of the phenomena occurring, the present invention is defined by the spatial-physical configuration of the proposed security elements and not to the given explanation of the phenomena by excitation of volume or surface plasmons or the occurrence of Resonance phenomena bound.

In the context of the invention, it is preferred if the feature range of the security element selectively influences incident electromagnetic radiation in the visible spectral range. In particular, the feature area can selectively reflect and / or transmit incident electromagnetic radiation. For example, the feature area can reflect certain spectral components of the visible light and transmit other spectral components of the visible light and thus appear with different colors in reflection and transmission.

To form a see-through security element, the feature area can in particular be made transparent or translucent. In the case of security elements that are designed to be viewed in reflection, the feature area or the substrate of the security element can also be opaque.

The feature area can contain different metallic nanostructures in different partial areas, for example in order to produce different colored areas within the security element.

In the context of the invention, the carrier medium is preferably formed by a transparent or colored lacquer layer.

The feature area contains, as metallic nanostructures, one or more subwavelength gratings with grating periods below the wavelength of visible light. The subwavelength gratings are designed as binary structures that contain only flat metallic surface sections at only two different height levels.

The subwavelength gratings can also be combined with a diffraction structure that spectrally splits the incident electromagnetic radiation. For the spectral broadening of the resonances that occur, the subwavelength gratings can have grating lines of varying width. According to the invention, laterally different color impressions are generated by subwavelength gratings which have a lateral variation of the grating profiles, namely a lateral variation of the profile depths. In this way, any color images are introduced into the security elements, for example screened color images which consist of a large number of small and differently colored pixel elements.

According to the invention, the security element contains a colored image made up of a plurality of pixel elements, the grid profiles within a pixel element being constant and in which the grid profiles are of different colors Pixel elements are designed differently in accordance with the respective desired color impression.

The generation of color images by means of subwavelength gratings is particularly suitable for diagonally metallically vapor-deposited dielectric gratings which show different colors in transmission and reflection, as explained in more detail below. Due to the asymmetrical grating profile, an asymmetry of the color appearance in the viewing angle in transmission or in reflection can usually be observed. The lateral variation of the grating profile can in particular consist of a lateral variation of the trench depth of the metallized dielectric grating. In addition to binary structures, obliquely vaporized asymmetrical multilevel profiles with laterally different depths can also be considered.

In order to generate subwavelength gratings with different profile depths, the following procedure can be used, for example: First, photoresist is applied to a grating substrate with a laterally constant trench depth, so that the trenches are completely filled. The substrate with the applied photoresist is then exposed to laser radiation of different intensity laterally and the trenches are partially exposed by removing the exposed photoresist.

The underlying physical effect for color generation is in particular polarization conversion through resonance excitation on gratings, which leads to selective transmission or reflection when a subwavelength grating is arranged between two crossed polarizers.

The grating periods of the subwavelength gratings are preferably between 10 nm and 500 nm, preferably between 50 nm and 400 nm and particularly preferably between 100 nm and 350 nm.

The subwavelength gratings can be formed by linear, one-dimensional gratings or also by two-dimensional cross gratings which are periodic in one or two spatial directions. In a further variant, the subwavelength gratings are formed by one or two-dimensional repeated arrangement of metallic structural elements, the structural elements being in particular in the form of squares, rectangles, circular areas, ring structures, strips or a combination of these elements or any other shape. Other possible shapes are, in particular, spheres, rhombuses or rods, but also strongly asymmetrical shapes such as open rings. All the arrangements mentioned can be periodic in one or two spatial directions.

In addition to one- or two-dimensional linear grids, one- or two-dimensional curved grids can also be provided according to the invention. With these curved grids, the azimuth angle of the grid lines changes continuously without abrupt jumps. The azimuth angle indicates the local angle between the grid lines (more precisely a tangent to the grid lines) and a reference direction, i.e. it describes the local orientation of the grid lines in the plane.

The subwavelength gratings can be integrated into an interference layer system in order to modify or enhance their optical effect.

In all variants of the invention, the feature area can be in the form of patterns, characters or a code.

Due to the small size of the metallic nanostructures, they can be used with particular advantage in security elements whose feature areas contain microstructures with a line thickness between approximately 1 μm and approximately 10 μm. Examples of such security elements are micro-optical moiré magnification arrangements, as they are in the publications DE 10 2005 062132 A1 and WO 2007/076952 A2 are described, micro-optical magnification arrangement of the moiré type, as described in the applications DE 10 2007 029 203.3 and PCT / EP2008 / 005173 are described, as well as modulo magnification arrangements, as they are in the application PCT / EP2008 / 005172 are described. All of these micro-optical magnification arrangements contain a motif image with microstructures which, when viewed with a suitably coordinated viewing grid, reconstructs a predetermined target image. As explained in more detail in the above-mentioned documents and applications, a large number of visually attractive magnification and movement effects can be generated, which lead to a high recognition value and a high level of protection against forgery of the security elements produced.

In an advantageous development of the invention, the microstructures for this purpose form a motif image which is divided into a plurality of cells in which each of the mapped areas of a predetermined target image are arranged. The lateral dimensions of the areas shown are preferably between approximately 5 μm and approximately 50 μm, in particular between approximately 10 μm and approximately 35 μm. In the case of the above-mentioned first-mentioned micro-optical moiré magnification arrangements, the depicted areas of the cells of the motif image each represent reduced images of the predefined target image which are completely accommodated within a cell. In the case of the micro-optical magnification arrangements of the moiré type, the depicted Areas of several spaced apart cells of the motif image taken together each represent a reduced image of the target image, the extent of which is larger than a cell of the motif image. In the most general case, the magnification arrangement represents a modulo magnification arrangement in which the mapped areas of the cells of the motif image each represent incomplete sections of the specified target image mapped by a modulo operation.

The security element preferably also has a viewing grid made up of a plurality of viewing grid elements for reconstructing the predetermined target image when viewing the motif image with the aid of the viewing grid. The lateral dimensions of the viewing grid elements are advantageously between approximately 5 μm and approximately 50 μm, in particular between approximately 10 μm and approximately 35 μm.

In the special case of a micro-optical moiré magnification arrangement, a motif image from a planar periodic or at least locally periodic arrangement of a plurality of micromotif elements is preferably applied as the microstructure. The lateral dimensions of the micromotif elements are advantageously between approximately 5 μm and approximately 50 μm, preferably between approximately 10 μm and approximately 35 μm. In addition, the opposite side of the carrier is expediently provided with a planar periodic or at least locally periodic arrangement of a plurality of microfocusing elements for the moiré-enlarged observation of the micromotif elements of the motif image. In some configurations it is advisable to arrange the microfocusing elements and the micromotif elements on the same side of the carrier. Also bilateral designs in which a micromotif element arrangement is replaced by two opposite Microfocusing element arrays can be considered infage.

The invention also includes a method for producing a security element of the type described.

One or more subwavelength gratings with grating periods below the wavelength of visible light are applied to a substrate as metallic nanostructures. For this purpose, for example, a relief structure in the form of the desired subwavelength gratings can be embossed into an embossed lacquer layer and a metallization can be applied, in particular vapor-deposited, to this relief structure. The metallization is expediently vapor deposited at a vapor deposition angle Q which is between 0 ° and 90 °, preferably between 30 ° and 80 °. The metallized relief structure is then advantageously covered with a further lacquer layer.

A one- or two-dimensional repeated arrangement of metallic structural elements can also be applied to the substrate, in particular vapor-deposited, as a sub-wavelength grating, as described in more detail below.

In a further advantageous production method, the nanostructures are produced by laser irradiation of a thin metal layer. The metal layer can be arranged on structured or unstructured areas of a substrate and either be exposed or embedded. The metal layer can be full-area and bombarded with a laser over the full area, or it can only be formed in certain areas, so that the laser irradiation leads to the formation of nanostructures only in the metallized and illuminated areas. In a further embodiment For example, a full-surface metal layer can only be illuminated vertically or at an angle at predetermined points with laser radiation, for example the radiation of a focused laser, so that nanostructures only arise at the illuminated points.

The invention also contains a security paper for the production of documents of value or the like as well as a data carrier, in particular a document of value such as a bank note, a passport, a certificate, an identity card or the like. According to the invention, the security paper or the data carrier is equipped with a security element of the type described. The security element can, in particular when it is present on a transparent or translucent substrate, also be arranged in or above a window area or a continuous opening in the security paper or the data carrier.

Further exemplary embodiments and advantages of the invention are explained below with reference to the figures. For the sake of clarity, a representation true to scale and proportion is dispensed with in the figures.

The ones in the Figures 2 , 3, 4, 5 , 6th , 7 and 9 The exemplary embodiments shown do not fall under the scope of protection of the appended claims, but serve for understanding the invention.

Fig. 1

eine schematische Darstellung einer Banknote mit einem Durchsichtssicherheitselement und einem aufgeklebten Transferelement, jeweils nach Ausführungsbeispielen der Erfindung,

Fig. 2

ein nicht erfindungsgemäßes Durchsichtssicherheitselement im Querschnitt,

Fig. 3 bis 5

nicht erfindungsgemäße Ausführungsbeispiele mit strukturierten Oberflächen zur Steuerung der räumlichen Verteilung der metallischen Nanopartikel,

Fig. 6

in (a) bis (c) Aufsichten auf Merkmalsbereiche weiterer Sicherheitselemente,

Fig. 7

ein nicht erfindungsgemäßes Ausführungsbeispiel, bei dem metallische Nanopartikel in ein Dünnschichtelement mit Farbkippeffekt integriert sind,

Fig. 8 und 9

schematische Querschnitte durch ein erfindungsgemäßes Sicherheitselement gemäß der Figur 8 mit Subwellenlängengitter und durch ein nicht erfindungsgemäßes Sicherheitselement gemäß der Figur 9 mit Subwellenlängengitter,

Fig. 10

stark schematisiert die Farbigkeit bestimmter, erfindungsgemäßer Subwellenlängengitter in Abhängigkeit von dem Bedampfungswinkel Q, wobei in (a) die Farbigkeit in Reflexion und in (b) die Farbigkeit in Transmission, jeweils in der nullten Beugungsordnung gezeigt ist,

Fig. 11

ein erfindungsgemäßes Sicherheitselement, dessen Merkmalsbereich mit einer metallisierten Prägestruktur mit zwei überlagerten Gittern versehen ist,

Fig. 12

eine schematische Aufsicht auf einen Merkmalsbereich mit einem in zwei Raumrichtungen periodischen, rechteckigen Kreuzgitter,

Fig. 13

in (a) und (b) Aufsichten auf Subwellenlängengitter, die aus zweidimensionalen periodischen Anordnungen von Strukturelementen gebildet sind,

Fig. 14

ein in ein Interferenzschichtsystem integriertes Subwellenlängengitter,

Fig. 15

in (a) bis (c) drei Ausführungsformen von Mikromotivelementen, die durch Füllung mit metallischen Nanostrukturen farbig erscheinen, und

Fig. 16

ein Ausführungsbeispiel wie in Fig. 15, bei dem sowohl die Mikromotivelemente als auch der umgebende Velinbereich nanostrukturiert sind.

Show it:

Fig. 1

a schematic representation of a banknote with a see-through security element and a glued-on transfer element, each according to exemplary embodiments of the invention,

Fig. 2

a see-through security element not according to the invention in cross section,

Figures 3 to 5

Embodiments not according to the invention with structured surfaces for controlling the spatial distribution of the metallic nanoparticles,

Fig. 6

in (a) to (c) supervision of feature areas of further security elements,

Fig. 7

an exemplary embodiment not according to the invention in which metallic nanoparticles are integrated into a thin-film element with a color-shift effect,

Figures 8 and 9

schematic cross-sections through a security element according to the invention according to FIG Figure 8 with a subwavelength grating and by a security element not according to the invention according to FIG Figure 9 with subwavelength grating,

Fig. 10

The color of certain subwavelength gratings according to the invention is strongly schematized as a function of the vapor deposition angle Q, whereby in (a) the color in reflection and in (b) the color in transmission, in each case in the zeroth order of diffraction,

Fig. 11

a security element according to the invention, the feature area of which is provided with a metallized embossed structure with two superimposed grids,

Fig. 12

a schematic plan view of a feature area with a rectangular cross grating that is periodic in two spatial directions,

Fig. 13

in (a) and (b) plan views of subwavelength gratings, which are formed from two-dimensional periodic arrangements of structural elements,

Fig. 14

a sub-wavelength grating integrated into an interference layer system,

Fig. 15

in (a) to (c) three embodiments of micromotif elements that appear colored when filled with metallic nanostructures, and

Fig. 16

an embodiment as in Fig. 15 , in which both the micromotif elements and the surrounding vellum area are nano-structured.

The invention will now be explained using the example of security elements for bank notes. Fig. 1 shows a schematic representation of a bank note 10, which is provided with two security elements 12 and 16 according to embodiments of the invention. The first security element represents a see-through security element 12, which is arranged over a see-through area 14, for example a window area or a continuous opening, of the bank note 10. The second security element 16 is formed by an opaque, glued-on transfer element of any shape.

Both security elements have metallic nanostructures in a feature area in which incident visible light excites volume or surface plasmons or resonance effects that generate novel color effects that are difficult to forge due to the small size of the respective coloring nanostructures.

As already explained above, plasmons represent the eigenmodes of collective vibrations of the free electrons relative to the ion cores in metals, which can be excited by incident electromagnetic radiation. At a certain wavelength, the freely movable charge carriers are excited to resonant oscillations, so that the light of this wavelength is preferentially absorbed and scattered in all spatial directions. In contrast, radiation with wavelengths outside the resonance range can pass largely undisturbed.

As a result of this effect, the metallic nanostructures according to the invention appear in transparency with a color impression that results from the wavelengths of the unaffected, non-resonant portion of the incident light. When viewed in reflection, in which the scattered light dominates the visual appearance, the color impression of the nanostructures, on the other hand, is mainly determined by the resonant portion of the spectrum. Which wavelengths can excite the resonant plasma oscillations depends not only on the material from which the nanostructures are made, but also on the shape and size of the nanostructures and the embedding medium.

The embodiment not according to the invention of Fig. 2 shows first of all a see-through security element 20 with a substrate 22 and a feature area, which is formed by a feature layer applied over the entire surface 24 is formed. The feature layer 24 contains a multiplicity of metallic nanoparticles 28 which are embedded in a carrier medium 26. Such a feature layer 24 can be produced, for example, by printing on a transparent lacquer 26 in which prefabricated metallic nanoparticles 28 with desired properties are dissolved.

The nanoparticles 28 have a diameter below the wavelength of visible light, preferably between 300 nm and 5 nm and in particular between 200 nm and 10 nm. In a preferred variant of the invention, the nanoparticles 28 are gold or silver particles. However, other metals such as copper, aluminum, nickel, chromium, tungsten, vanadium, palladium, platinum or alloys of these metals, albeit partially in a weakened or modified form, show color effects due to plasmon excitation, so that these metals or metal alloys are also considered to be Material for the nanoparticles 28 come into consideration.

In addition to spherical nanoparticles 28, it is also possible to use particles of other shape, such as ellipsoids of revolution, any polyhedron or rod-shaped or platelet-shaped particles. Particles deviating from the spherical shape show, if they are oriented in a preferred direction in space, additional effects that are dependent on the polarization direction of the incident light.

In addition to homogeneous metallic nanoparticles 28, coated core-shell particles can also be used for color generation. These can have both a metallic core with a dielectric or metallic shell and a dielectric core with a metallic sheath. Examples of such designs are silver particles with a TiO ₂ shell or polystyrene cores with a gold coating. There is hardly any limit to the number of possible combinations, especially since the materials can also be in crystalline or polycrystalline form in addition to the amorphous phase.

In the simplest case, the transparent lacquer 26, in which the nanoparticles 28 are dissolved, is applied over the entire surface to the substrate 22, for example printed on, as in FIG Fig. 2 shown. Broadband incident light 30 then excites specific plasma oscillations (plasmons) in the nanoparticles 28 depending on the material, shape and size of the particles 28 and their embedding medium 26. For example, the resonance frequency for essentially spherical gold particles with a diameter of 50 nm is approximately 520 nm, for gold particles with a diameter of 150 nm it is approximately 580 nm.

In the embodiment of Fig. 2 the nanoparticles 28 and the embedding medium 26 are matched to one another in such a way that the resonance frequency of the embedded nanoparticles 28 is green at a wavelength of approximately 530 nm. When viewed in reflection 32, where the light scattered by the nanoparticles 28 dominates the color impression, the feature layer 24 therefore appears green. In transmission 34, however, the feature layer 24 appears in the subtractive complementary color, that is to say with a red color impression.

In contrast to periodic diffraction structures or interference layer systems, the color impression of the metallic nanoparticles does not depend on the angle of incidence of the radiation and the viewing direction. The security elements according to the invention also do not run through the visible spectrum or sections thereof when tilted, but essentially have one constant color impression. Since the color effects are caused by nanostructures that are much smaller than the period of conventional diffraction gratings, they are particularly resistant to forgery, since such small structures can hardly be produced using conventional methods such as direct exposure or dot-matrix methods.

Instead of being designed over the full area, the feature area of the security element 20 can also be designed in the form of patterns, characters or a code. It is also possible to provide different metallic nanostructures in different partial areas of the feature area, for example nanoparticles 28 made of different materials and / or nanoparticles 28 of different shapes and sizes. This allows different areas of the feature area to be colored differently.

In addition, the lacquer 26 provided with the coloring nanoparticles 28 can additionally contain conventional color or effect pigments in order to modify the observable color effects. Different types of metallic nanoparticles 28, for example with varying diameters, can also be mixed with one another in order to produce a desired color effect in cooperation.
In a further embodiment, measures can be taken to influence the spatial distribution of nanoparticles 28 initially homogeneously dispersed in a carrier medium or the preferred direction of non-spherical nanoparticles. This can be done, for example, by equipping the nanoparticles with a magnetic core so that they can be concentrated at the intended locations of the feature area with the aid of spatially varying magnetic fields. The nanoparticles 28 are initially still movable in the carrier medium 26. Only after If they have been placed and / or aligned with the aid of the magnetic field, they are immobilized in that the binding agent of the carrier medium 26 is cured, for example by drying or irradiation with UV light, or the carrier medium 26 or at least the solvent contained therein is evaporated by supplying heat.

Functionalized surfaces of nanoparticles offer additional possibilities to influence the arrangement of the nanoparticles. For example, through a suitable functionalization of the surface, it can be achieved that the particles are arranged at a certain distance and / or in a defined grid. In addition, a suitably selected functionalization can prevent clustering of the nanoparticles.

Functionalization of the substrate surface can also serve to arrange and periodically align the nanoparticles. By functionalizing the substrate surface and possibly also the surface of the nanoparticles, these can be deposited in a targeted manner on predefined areas of the substrate. This makes it possible, for example, to arrange the nanoparticles on grating lines in order to influence the diffraction property of the grating, for example to increase it.
Alternatively, non-magnetic nanoparticles 28 can also be coupled to magnetic carrier particles by functional coatings, which are then specifically arranged and / or aligned together with the coloring nanoparticles 28 by external magnetic fields.

According to a preferred variant of the invention, the distribution of the nanoparticles 28 is specifically influenced by structuring the surface to which they are applied. As in the exemplary embodiment not according to the invention the Fig. 3 As shown, for example, a transparent UV-curing lacquer layer 40 can be provided with a desired relief embossing in a manner known per se, so that a structured surface with elevations 42 and depressions 44 is created. A fluid medium 46 in which the nanoparticles 48 are dissolved is then applied, for example printed, to the surface structured in this way. Then the fluid medium 46 is knife-coated or wiped off the coated surface, so that the nanoparticles 48 only remain in the depressions 44, but not on the raised surface regions 42.

In order to prevent the nanoparticles 48 from falling out of the depressions 44 during further processing, the structure can be covered with a further lacquer layer, not shown in the figures. If the lacquer used for covering flows around the nanoparticles 48, the refractive index of the medium embedding the particles can also be defined in this way. At present, however, it is preferred that the nanoparticles 48 remain embedded in the original carrier medium 46, which remains in the depressions 44 together with the nanoparticles 48 when the surface is doctored off.

The in Fig. 4 The exemplary embodiment shown, not according to the invention, is additionally provided with a metal layer 50 between substrate 22 and UV lacquer layer 40 in order to modify the color impression of the nanoparticles 48 in a targeted manner. Alternatively, as in Fig. 5 shown, before the application of the nanoparticles 48, a metal layer 52 is applied to the embossed UV lacquer layer 40, for example by vapor deposition, thereby modifying the color impression of the nanoparticles 48.

According to an advantageous manufacturing variant, that in the international patent application PCT / EP2007 / 005200 microtogravure technology described are used, which combines the advantages of printing and embossing technologies. Briefly summarized, microtogravure printing technology provides a mold, the surface of which has an arrangement of elevations and depressions in the form of a desired microstructure. The depressions of the tool mold are filled with a curable colored or colorless varnish containing the nanoparticles, and the carrier to be printed is pretreated for good anchoring of the varnish. The surface of the tool mold is then brought into contact with the carrier and the lacquer in contact with the carrier is hardened in the depressions of the tool mold and thereby bonded to the carrier. The surface of the tool mold is then removed from the carrier again, so that the hardened lacquer with the nanoparticles connected to the carrier is drawn out of the depressions in the tool mold. For a more detailed description of the microtogravure printing process and the advantages associated with it, please refer to the patent application mentioned PCT / EP2007 / 005200 referenced, the disclosure content of which is included in the present application.

In the case of the security elements described above, the visual impression can not only be generated by the effects of the plasmon excitation in the nanoparticles 48, but can also be influenced by diffraction effects on the structures that are predetermined by the elevations 42 and depressions 44. In the case of periodically arranged linear trenches, for example, in addition to the plasmon effects described, a spectral splitting of the light that is typical for diffraction on a linear grating can be seen. These diffraction effects can be specifically integrated into the design of the security element. Are such in other designs Strongly color-producing additional effects are undesirable, so the elevations and depressions 42, 44 can also be arranged irregularly and color phenomena caused by diffraction can be largely suppressed.

For illustration shows Fig. 6 (a) a plan view of the feature area 60 of a security element not according to the invention, in which the depressions 44 with the nanoparticles 48 are periodically arranged in two spatial directions. It goes without saying that the period lengths denoted by px and py can be the same or different, so that the same or different diffraction color effects occur in the x-direction and y-direction.

When looking at the feature area 62 of the Fig. 6 (b) the depressions 44 with the nanoparticles 48 are only arranged periodically in the y-direction, while they are randomly distributed in the x-direction. Diffraction effects due to the periodic arrangement of the depressions 44 occur with such a design only in the y-direction, while they are suppressed in the x-direction. If the color-splitting diffraction effects are to be completely suppressed, the depressions 44 can also be arranged randomly in both spatial directions, as in the feature area 64 of FIG Fig. 6 (c) shown.

Fig. 7 shows an embodiment 70 of a further variant not according to the invention, in which the nanoparticles 78 are integrated in a thin-film element 72 with a color-shift effect. For this purpose, a reflective metal layer 74, for example an aluminum layer with a thickness of at least 10 nm, a dielectric intermediate layer 75 made of a UV-curable material and a semitransparent absorber layer 76, which can be formed, for example, by an approximately 8 nm thick chrome layer, is applied to a substrate 22 can. The intermediate dielectric layer 75 is preferably formed from a carrier medium with a high refractive index. It also contains the desired metallic nanoparticles 78, which can be achieved, for example, in that the nanoparticles 78 are admixed with the intermediate layer material before application. Overall, in the case of the security element 70 designed for viewing in reflection, the filter effect of the nanoparticles 78 is combined with the color filter effect of the color-shifting thin-film system 72.

In some configurations, the semitransparent absorber layer 76 can also be dispensed with. If the security element 70 is to be used in transmission, that is to say for example in the see-through window of a bank note, then the lower metal layer 74 is expediently designed to be semitransparent. It goes without saying that the feature area also in the exemplary embodiments of Figures 3 to 7 can be designed in the form of patterns, characters or a coding and that here too different metallic nanostructures can be provided in different partial areas. Both transparent and non-transparent layer systems can be used as the substrate 22. In particular, the substrate 22 can be formed, for example, by a transparent or opaque plastic film that remains in the finished security element or by a transfer film that is peeled off after the security element has been transferred to the bank note 10. The substrate 22 can also be formed by the bank note paper itself. For this purpose, the nanoparticles can, for example, be suspended in a primer before printing and printed directly onto the banknote paper.

The production of the metallic nanoparticles themselves can take place by physical or chemical processes known to the person skilled in the art. A physical process is, for example, laser ablation.

Instead of using prefabricated nanoparticles that are dissolved in suitable media and applied to a desired substrate, for example by printing, one or more subwavelength gratings can also be applied directly to the substrate of the security element according to the invention. Such periodic nanostructures on the one hand allow stronger color effects than the previously described metallic nanoparticles, on the other hand the large number of degrees of freedom in the manufacture of counterfeit security of such security elements increases further.

In the case of subwavelength gratings, extraordinary changes in intensity can occur in the transmission or in the reflection if the incident light leads to resonances in the spaces or in the cavities of the grating structure. In the case of transmission gratings, a strong redistribution of intensity between reflection and transmission can be observed for certain wavelength ranges. These so-called cavity resonances also lead to increased absorption of light. It is worth mentioning that this effect can also cause an extraordinary increase in transmission.

The so-called Wood anomalies also influence the transmission and reflection spectra of gratings in the zeroth diffraction order regardless of the polarization of the incident light. A Wood anomaly is associated with the creation of a new diffraction order, that is, it occurs when the angle of reflection is 90 °. The spectral positions of the Wood anomalies can thus be derived from the lattice equation. They result for wavelengths λ = (p / m) (1 ± sin a), where p is the grating period, a is the angle of incidence and m is the order of diffraction. When a diffraction order disappears, its intensity has to be redistributed to the remaining diffraction orders, which also results in a spectral one Changes in intensity in the zeroth diffraction order leads. Finally, an increase in transmission, accompanied by a decrease in reflection, was observed in wire grids for wavelengths of the Wood anomalies under TE polarization (E vector parallel to the grating structure). The transmission is reduced for increasingly larger wavelengths and ultimately tends to zero in the limit case.

To illustrate this, structures are first described that have a periodicity only in one dimension. Fig. 8 shows a cross section through a security element 80 with a transparent carrier film 82, on which a UV embossing lacquer layer 84 is printed and embossed in the form of a rectangular profile, which has a period length p, for example 300 nm, a web width b, for example 100 nm, and a pitch h , for example 100 nm. An aluminum layer 86 with a thickness d, for example 30 nm, was then vapor-deposited vertically onto the embossing lacquer layer 84, and the resulting structure was provided with a further protective lacquer layer 88.

This results in a metallic binary structure 86 embedded in the lacquer layers 84, 88, which contains exclusively flat metallic surface sections at only two different height levels (metallic bi-grating). According to variants not falling under the scope of protection, the metallic surface sections can also be arranged on more than two height levels, in particular on n = 3 to n = 16 different height levels, and thus form a more general multilevel structure.

If the vapor deposition angle Q of the metal layer 90 deviates from 90 °, a subwavelength grating with a z-shaped metal profile is created according to a variant not falling under the scope of protection, as in FIG Fig. 9 illustrated for the case Q = 45 °. In the simplified representation of the Fig. 9 is in it is assumed that the width of the metal application in the lower level is predetermined by the geometric shading during vapor deposition and that the thickness d of the metal film 90 is identical on the upper and lower levels. The regions 92, 94 and 96 below, inside and above the z-shaped metal profile can generally have different refractive indices ni, n ₂ and n ₃ , respectively. When using standard UV lacquer for the embossed lacquer layer and the protective lacquer layer, however, these values are generally all n = 1.5.

The transmission and reflection spectra of such subwavelength gratings can be calculated, for example, with the aid of electromagnetic diffraction theories. In order to be able to estimate the perceived color of these grids, the spectrum calculated for the visible wavelength range is convoluted with the spectrum of the standard lamp D65 and the sensitivity curves of the human eye. This results in the parameters X, Y and Z, which reflect the color values red, green and blue.

Fig. 10 shows in a highly schematic manner the color of subwavelength gratings according to the invention with a grating period p = 300 nm, a ridge width b = 100 nm, a pitch h = 100 nm, a thickness d = 30 nm of the vapor-deposited aluminum layer and the same refractive indices of the surrounding dielectrics n ₁ = n ₂ = n ₃ = 1.5 for normal incidence of unpolarized light. In Fig. 10 (a) the color values X (curve 100-R), Y (curve 102-R) and Z (curve 104-R) of the reflected light in the zeroth order of diffraction as a function of the vapor deposition angle Q are shown. Fig. 10 (b) shows the color values X (curve 100-T), Y (curve 102-T) and Z (curve 104-T) of the transmitted light also in the zeroth order of diffraction.

The in Fig. 8 The special case of vertical vapor deposition shown is for Q = 90 °. With an increasingly inclined steaming angle, a Z-shaped wire profile emerges, whereby the in Fig. 9 shown profile for Q = 45 ° results. The degree of coverage of the metal film becomes smaller and the transmission of light increases. If the angle Q is smaller than arctan (h / (pb)), there is no longer any metallization of the lower level.

A nano-structure becomes very colorful if one of the color values X, Y, Z is dominant compared to the other color values or if the color values differ greatly from one another. Like the curves 100, 102 and 104 of FIG Fig. 10 As can be seen, especially for vapor deposition angles Q in the range between about 45 ° and about 80 °, the color value Z dominates the transmission ( Fig. 10 (b) , Curve 104-T), while the color values X and Y dominate the reflected radiation ( Fig. 10 (a) , Curves 100-R, 102-R). Such subwavelength gratings thus appear with a clearly pronounced color in transmission and reflection.

For color perception it is also desirable that the reflection of an object is at least 20% so that the color spectrum reflected on the object stands out from the reflected light of the surrounding medium. In contrast, the transmission can be lower for color perception, since usually only the transmitted light of the object is observed and the scattered light of the surroundings is covered. For the light intensity of the grating described above, a reflection of 30% to 60% and a transmission between 5% and 45% are obtained for an evaporation angle Q in the range between 30 ° and 90 °. In the case of inclined vaporization angles, the transmission increases while the reflection is reduced.

In addition to the effects described, the color effect of the subwavelength gratings according to the invention changes when viewed in polarized light. This is also how the color-imparting feature areas according to the invention differ from colored surfaces that were produced with conventional means. For example, for subwavelength gratings with the above-mentioned grating parameters, in particular the intensity of the color value Z (blue) changes with the polarization of the incident light, with the differences between TE polarization (E vector of the incident light parallel to the grating lines) and TM polarization ( E-vector of the incident light perpendicular to the grid lines) are particularly large at an evaporation angle in the range of Q = 45 °.

Due to the asymmetrical grating profile, an asymmetry of the color appearance in the viewing angle in transmission or in reflection can also be observed. By means of targeted lateral variation of the grating profiles, in particular the profile depth, it is therefore possible to generate laterally different color impressions within the security element and thus also color images, as described in more detail above.

In further exemplary embodiments of the invention, the described subwavelength gratings can be combined with a diffraction structure which spectrally splits incident electromagnetic radiation. For illustration shows Fig. 11 a security element 110, the feature area of which is provided with a metallized embossed structure 112 with two superimposed grids. The grating with the smaller grating period p _s forms a sub-wavelength grating of the type described above. This sub-wavelength grating is superimposed with a second grating of a significantly larger period p ₁ , which is used to generate a multiplication or spectral broadening of the resonances of the sub-wavelength grating described above.

If locally varying widths of the metallic grid lines are used, for example a modulation of the grid line width in the form of a beat or a statistical variation of the grid line widths, the plasmon resonances can be spectrally broadened. As a result, a broader range of the visible light spectrum can be influenced in its intensity than would be the case with a strictly periodic grating.

As a generalization of the one-dimensional subwavelength gratings described so far, two-dimensional cross gratings can also be used, which are periodically or also statistically arranged in one or two spatial directions. Fig. 12 shows a schematic plan view of a feature area 120 with a rectangular cross grid 122 which is periodic in two spatial directions. The sequence of hatched and non-hatched rectangles 124, 126 each represents higher or lower lying metallized surface sections, as shown in cross section, for example in FIG Fig. 8 are shown.

Due to the rectangular design of the cross grating 122, the period lengths in the x-direction and y-direction, px and py, are generally different. With different period lengths px, py, the cross grating 122 produces a different color impression in the polarized light, depending on whether the light is polarized vertically or horizontally. When viewing with unpolarized light, the viewer perceives a mixed color. If, on the other hand, the period lengths px and py are the same, the cross grating looks the same when viewed with unpolarized light as when viewed with vertically or horizontally polarized light.

The one- or two-dimensional sub-wavelength gratings can also be formed by a repeated arrangement of metallic structural elements, In addition to square or rectangular elements, circular, elliptical, ring-shaped or any shaped elements can also be considered.

Fig. 13 For illustration, in (a) shows a plan view 130 of a subwavelength grating which is formed from a two-dimensional periodic arrangement of ring elements 132. The period lengths px and py are both below the wavelength of visible light and can be 300 nm, for example. While in Fig. 13 (a) the case px = py is shown, the period lengths can of course also be different. For the excitation of plasmons by incident light, the ring width of the ring elements 132 is of particular importance.

When supervising 134 the Fig. 13 (b) two different geometries are combined with one another, namely strip-shaped structural elements 136 and ring-shaped structural elements 132. In particular, the strips 136 are excited by the external electromagnetic radiation. They transport the absorbed electromagnetic energy to the ring elements 132 and partially transfer it to them. Since structural elements of different geometry generally also have different plasmon resonances, such a combination of different structural elements can lead to a modified resonance behavior and thus to a changed color impression of the overall system.

In general, the arbitrarily shaped elements can be distributed statistically or stochastically on the surface that is to appear colored.

It goes without saying that the variants described for the one-dimensional subwavelength gratings, in particular the use of Wood anomalies and the combination of the subwavelength gratings with diffraction gratings can also be used with two-dimensional cross gratings and the one- or two-dimensional structure element arrangements.

The described subwavelength gratings can also be integrated into an interference layer system in order to modify or enhance their optical effect. An exemplary layer system is shown in the cross section of Fig. 14 shown. In this case, a UV embossing lacquer layer 142 is printed on a transparent carrier film 140 and embossed in the form of a desired one- or two-dimensional subwavelength grating. An aluminum layer 144 of a desired thickness is then vapor-deposited onto the embossing lacquer layer 142 perpendicularly or at a certain vapor deposition angle Q.

A layer 146 with a high refractive index, preferably ZnS or TiO ₂ , is then applied, for example likewise by vapor deposition. Whether or how clearly the embossed structure can still be found on the surface of this high-index layer 146 depends on the circumstances under which the layer was applied. The most important parameter in this regard is of course the layer thickness. The interference layer system is completed by applying a further layer 148 of a transparent material with a lower refractive index, for example protective lacquer with n = 1.5. The optical effect of the high-index dielectric layer 146 is essentially determined by its thickness and the difference in refractive index to the surroundings.

The high resolution required for the described subwavelength gratings can be achieved, for example, with the aid of electron beam lithography systems, with even the smallest particles with a lateral extent of a few 10 nm still being produced with individual outlines can. PMMA is typically used as the resist. The origination by means of electron beam lithography is followed by galvanic molding and the production of embossing tools, with the help of which the nanostructures can then be reproduced by embossing in UV-curable lacquer or a thermoplastically deformable plastic on film webs. The metallic nanostructures are obtained in the next step by vapor deposition or sputtering with the appropriate material in the desired layer thickness, whereby it should be noted that the metal layer thickness should generally be smaller than the embossing depth. The metals used are preferably gold, silver, copper and aluminum.

A particular advantage of the metallic nanostructures according to the invention is that they can be arranged in a sufficient number of periods or quasi-periods even in small microstructures with dimensions of a few micrometers. Typical examples of such microstructures are letters and symbols that form the micromotif images of a moiré magnification arrangement. The mode of operation and advantageous arrangements for such moiré magnification arrangements are given in the publications DE 10 2005 062 132 A1 and WO 2007/076952 A2 described, the disclosure content of which is included in the present application.
If such microstructures are filled with nanostructures according to the invention, they can be given a color that is difficult or impossible to achieve in any other way, in particular with several colors in a very small space.

Fig. 15 shows in (a) to (c) by way of example three embodiments of micromotif elements 150 which appear colored when filled with metallic nanostructures. The micromotif elements 150 shown in Fig. 15 for illustration are only represented by the letter "A", typically have a lateral dimension between 10 µm and 35 µm and a line width between 1 µm and 10 µm and can therefore be designed in color with conventional methods only with difficulty.

At the in Fig. 15 (a) As shown in the variant of the invention, the region of the micromotif elements 150 contains metallic nanoparticles 152 which are embedded in a carrier medium 154, as described in more detail above. The micromotif elements 150 of the Fig. 15 (b) are filled with a linear subwavelength grating 156, and the in Fig. 15 (c) micromotif elements 150 shown with a square cross grating 158.

The color generation or blackening is brought about by the excitation of plasmons in the respective nanostructures 152, 156, 158, as already described above. In the case of filling with the line grating 156, the period of which should be significantly smaller than the wavelength of visible light, a polarizing effect can also be observed in addition to the color effect. Which color is created in detail depends on the nature of the nanostructures and the type of dielectric embedding, as already explained in detail. The deterministic structures 156, 158 of the Figures 15 (b) and (c) can be produced by embossing in UV lacquer and subsequent vapor deposition of a metal layer of suitable thickness. If necessary, instead of a simple metal layer, a layer system can also be applied, as described above, for example in order to additionally intensify the plasmonic color effects.

In the case of the profile shapes that are created, the surface sections provided with nanostructures can be located on the level of the vellum area or can be offset downwards or upwards in comparison to this level. Typical Embossing depths are in the range between 10 nm and 500 nm for the nanostructures and up to a maximum of 10 µm for the microstructures.

In addition, the areas offset upwards or downwards, which define the surfaces of the micromotif elements 150, can also have curved profiles.

In the representations of the Fig. 15 The vellum area consists of an unstructured, smooth surface, while the surfaces forming the microstructures are equipped with nanostructures. However, the reverse case is also possible, in which the microstructures do not experience any additional structuring, but the surrounding vellum area is nanostructured. As in the embodiment of Fig. 16 As shown, a combination of both possibilities comes into consideration, in which both the micromotif elements 160 and the surrounding vellum area 162 are provided with nanostructures 164, 166, each of which achieves different color effects.

In addition to the configurations described so far, the nanostructures can also change within a microstructure, for example continuously, abruptly or statistically. The same applies to the nanostructure filling of the vellum area: it does not necessarily have to be homogeneous either, as in the exemplary embodiments of FIG Figures 15 and 16 shown. The surface sections that do not contain any nanostructures can also be unstructured or filled with other structures. For example, microstructures such as sawtooth structures or retroreflective cube-corner structures or so-called moth-eye structures that absorb light and therefore look dark to black are possible.

Claims (15)

1. A security element (80) for security papers, value documents and the like, having a feature region that selectively influences incident electromagnetic radiation and that includes metallic nanostructures in which volume or surface plasmons are excited and/ or resonance effects are caused by the incident electromagnetic radiation and the feature region includes, as metallic nanostructures, one or more subwavelength gratings having grating periods below the wavelength of visible light, characterized in that the subwavelength gratings are formed as binary structures (86) that include exclusively planar metallic areal sections on only two different height levels, the subwavelength gratings having a lateral variation in the grating profiles, specifically a lateral variation in the profile depths, in order to produce laterally differing color impressions, and the security element including, composed of a plurality of pixel elements, a colored image in which, in each case, the grating profiles are constant within a pixel element, and the grating profiles of different colored pixel elements are differently developed in accordance with the respective color impression.
2. The security element according to claim 1, characterized in that the feature region selectively influences incident electromagnetic radiation in the visible spectral range.
3. The security element according to one of claims 1 or 2, characterized in that the feature region selectively reflects and/or transmits incident electromagnetic radiation.
4. The security element according to at least one of claims 1 to 3, characterized in that the feature region is transparent or translucent.
5. The security element according to at least one of claims 1 to 4, characterized in that the feature region includes different metallic nanostructures in different sub-regions.
6. The security element according to at least one of claims 1 to 5, characterized in that the subwavelength gratings are combined with a diffraction structure that splits the incident electromagnetic radiation spectrally.
7. The security element according to at least one of claims 1 to 6, characterized in that the subwavelength gratings comprise grating lines having a varying width.
8. The security element according to at least one of claims 1 to 7, characterized in that the grating periods of the subwavelength gratings are between 10 nm and 500 nm, preferably between 50 and 400 nm and particularly preferably between 100 nm and 350 nm.
9. The security element according to at least one of claims 1 to 8, characterized in that the subwavelength gratings are formed by linear gratings, or the subwavelength gratings are formed by two-dimensional cross-line gratings that are periodic in one or two spatial directions, or the subwavelength gratings are formed by curved one- or two-dimensional gratings having a continually changing azimuth angle of the grating lines, or the subwavelength gratings are formed by repeated one- or two-dimensional arrangement of metallic structure elements.
10. A method for manufacturing a security element (80) according to at least one of claims 1 to 9, in which, in a feature region, the security element (80) is provided with metallic nanostructures in which volume or surface plasmons are excited and/or resonance effects are caused by the incident electromagnetic radiation, and as metallic nanostructures, one or more subwavelength gratings having grating periods below the wavelength of visible light are applied to a substrate, the subwavelength gratings being applied having a lateral variation in the grating profiles, specifically having a lateral variation in the profile depths.
11. The method according to claim 10, characterized in that a relief structure is embossed in an embossing lacquer layer (84) in the form of the desired subwavelength gratings, and a metalization (86) is applied to, especially evaporated onto, the relief structure.
12. A security paper for manufacturing security or value documents, such as banknotes, checks, identification cards, certificates or the like, that is furnished with a security element according to at least one of claims 1 to 9.
13. The security paper according to claim 12, characterized in that the security paper comprises a carrier substrate composed of paper or plastic.
14. A data carrier, especially a branded article, value document or the like, having a security element according to one of claims 1 to 9.
15. The data carrier according to claim 14, characterized in that the security element is arranged in or over a window region or a through opening in the data carrier.

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