DE102013009972A1 - security element - Google Patents

Abstract

The invention relates to a security element for the production of documents of value, such as banknotes, checks or the like, which has a substrate with a top side and provides at least one image (11), the image (11) being formed by a microcavity structure (4) which has a plurality of adjacent Has microcavities (5, 6), and a smooth area (7) is formed, the microcavities (4) each have a structure width (w) of 0.5 μm to 3 μm in a spatial direction parallel to the top and a structure depth (perpendicular to it) ( t) and have an aspect ratio that is defined by the ratio of structure depth (t) to structure width (w), and a metal-containing coating is applied to the microcavity structure (4) and the smooth area (7), the aspect ratio of the microcavities ( 5, 6) is greater than 0.3 and the metal-containing coating (9) is formed on the micro-cavity structure (4) so that the micro-cavity structure (4) and the smooth area (7) have a brightness difference of at least 10% in transmitted light so that the microcavity structure (4) forms a foreground and the smooth area (7) forms a background of the image (11).

Description

The invention relates to a security element for producing value documents, such as banknotes, checks or the like, which has a substrate with a top side and at least one image, the image being formed by a microcavity structure having a plurality of adjacent microcavities and a smooth region in that the microcavities each have a feature width of 0.5 μm to 3 μm and a structure depth perpendicularly thereto in a spatial direction lying parallel to the top side and have an aspect ratio which is defined by the ratio of structure depth to feature size, and on the microcavity structure and the smooth Area a metal-containing coating is applied.

The invention further relates to a value document with such a security element.

The invention also relates to a production method for a security element for documents of value, such as banknotes, checks or the like, which provides at least one image, wherein on a substrate having an upper side, to form an image, a microcavity structure having a multiplicity of adjacent microcavities, and a smooth region is formed, the microcavities are each formed in a spatial direction lying parallel to the upper side with a feature width of 0.5 .mu.m to 3 .mu.m and perpendicular to it with a structure depth and have an aspect ratio which is defined by the ratio of structure depth to structure width , And on the micro cavity structure and the smooth area a metal-containing coating is applied.

In the prior art, security elements are known which have a microcavity structure. They provide photomicrographs that are magnified with moiré magnification arrangements. The design of microimages for moiré magnification arrangements has already been discussed in many ways in the art.

Microcavity structures are also known to effect absorption of incident radiation thereby providing, for example, a background to an image. The EP 1434695 B1 describes an absorbent structure having a period less than the wavelength of light. The structure is constructed as a cross lattice with a sinusoidal profile. The WO 2005/106601 A2 relates to moiré magnification arrangements with microimages consisting of antireflecting areas and partially reflecting areas. The antireflective surface is formed by nanostructures with a period less than 700 nm and a depth between 150 and 350 nm. In the EP 1979768 A1 are microlens array multilayer bodies are described in which micro images are generated by micro-holes or by areas with different opacity. The WO 2002/101669 A2 describes microimages formed by fine dots or perforations. The EP 1476317 A1 as well as the US 7468842 B2 describe concave surfaces, picture elements as relief surfaces which are filled with color, and light trap patterns formed by sub-wavelength structures for generating microimages for moiré magnification arrangements.

From the DE 10 2008 046 128 A1 For example, a matte structure for a security element is known which has a multiplicity of microelements each having a lateral dimension of less than 50 μm, wherein at least one geometry parameter of the microelements varies randomly in order to produce the matte effect. The geometry parameter can be the depth of the microelements.

The WO 2005/095119 A1 describes a security element in the form of a multi-layered film body, which shows an angle-dependent color shift effect by means of interference. In this case, a relief structure is formed in a replication lacquer layer, which is designed with regard to its geometry in such a way that the color shift effect is produced only in individual sections. For this purpose, the aspect ratio of the relief structure is varied.

The DE 10 2006 050 047 A1 describes a see-through security element with microstructures formed as microcavities spaced 1 μm apart.

Metallized security elements which have an increased transmission in finely structured areas are known from US Pat EP 1786632 B1 known. It discloses a relief structure provided with a metal layer, which is designed as a diffractive structure and contains regions of sinusoidal linear gratings or of cross gratings which have a period in the sub-wavelength range. In order to achieve the desired transmission effect, the structural elements have an aspect ratio of greater than 0.5. The production of such structures, in particular also the replication and embossing of sub-wavelength structures with a high aspect ratio, is not without problems.

From the WO 2012/069163 A1 For example, a reflective security element is known that has a microcavity structure that functions as retroreflectors. By suitable structuring, a colored image is produced in plan view.

The invention is therefore based on the object to the disadvantages of the prior art and, more particularly, to provide an improved microcavity structured image security device that is less constrained in the manufacturing process and yet can provide a high resolution image recognizable in transmission.

• – das Bild durch eine Mikrokavitätenstruktur, die eine Vielzahl nebeneinanderliegender Mikrokavitäten aufweist, und einen glatten Bereich gebildet ist,
• – die Mikrokavitäten jeweils in einer zur Oberseite parallel liegenden Raumrichtung eine Strukturweite von 0,5 μm bis 3 μm und senkrecht dazu eine Strukturtiefe aufweisen und ein Aspektverhältnis haben, das durch das Verhältnis von Strukturtiefe zu Strukturweite definiert ist, und
• – auf die Mikrokavitätenstruktur und den glatten Bereich eine metallhaltige Beschichtung aufgebracht ist,
• – das Aspektverhältnis der Mikrokavitäten größer als 0,3 ist und
• – die metallhaltige Beschichtung so ausgebildet ist, dass Mikrokavitätenstruktur und glatter Bereich in Durchlichtbetrachtung einen Helligkeitsunterschied von mindestens 10% haben, so dass die Mikrokavitätenstruktur einen ersten Bildbereich und der glatte Bereich einen zweiten Bildbereich des Bildes bilden.

The object is achieved with a security element for the production of documents of value, such as banknotes, checks or the like, which has a substrate with a top and at least one image provides, wherein

• The image is formed by a microcavity structure having a multiplicity of adjacent microcavities and a smooth area,
• - The microcavities each have a feature width of 0.5 microns to 3 microns and perpendicular to a structure depth in a direction parallel to the top spatial direction and have an aspect ratio, which is defined by the ratio of structure depth to structure width, and
• A metal-containing coating is applied to the microcavity structure and the smooth area,
• The aspect ratio of the microcavities is greater than 0.3 and
• - The metal-containing coating is formed so that the micro-cavity structure and smooth area in transmitted light observation have a brightness difference of at least 10%, so that the micro-cavity structure form a first image area and the smooth area a second image area of the image.

• – an einem Substrat, das eine Oberseite aufweist, zur Erzeugung eines Bilds eine Mikrokavitätenstruktur, die eine Vielzahl nebeneinanderliegender Mikrokavitäten aufweist, und ein glatter Bereich ausgebildet wird,
• – die Mikrokavitäten jeweils in einer zur Oberseite parallel liegenden Raumrichtung mit einer Strukturweite von 0,5 μm bis 3 μm und senkrecht dazu mit einer Strukturtiefe ausgebildet sind und ein Aspektverhältnis haben, das durch das Verhältnis von Strukturtiefe zu Strukturweite definiert ist,
• – auf die Mikrokavitätenstruktur und den glatten Bereich eine metallhaltige Beschichtung aufgebracht wird,
• – das Aspektverhältnis der Mikrokavitäten größer als 0,3 ist und
• – die metallhaltige Beschichtung so ausgebildet wird, dass Mikrokavitätenstruktur und glatter Bereich in Durchlichtbetrachtung einen Helligkeitsunterschied von mindestens 10% haben, so dass die Mikrokavitätenstruktur einen ersten Bildbereich und der glatte Bereich einen zweiten Bildbereich des Bildes bilden.

The object is further achieved with a production method for a security element for value documents, such as banknotes, checks or the like, which provides at least one image, wherein

• A microcavity structure having a multiplicity of adjacent microcavities on a substrate having an upper side to form an image, and a smooth region being formed,
• The microcavities are each formed in a spatial direction lying parallel to the upper side with a feature width of 0.5 μm to 3 μm and perpendicular thereto with a structure depth and have an aspect ratio which is defined by the ratio of structure depth to feature size
• A metal-containing coating is applied to the microcavity structure and the smooth area,
• The aspect ratio of the microcavities is greater than 0.3 and
• The metal-containing coating is formed in such a way that the microcavity structure and the smooth region have a brightness difference of at least 10% when viewed through transmitted light, so that the microcavity structure forms a first image area and the smooth area forms a second image area of the image.

Finally, the object is also achieved with a value document having a security element of the type mentioned.

The increased transmission of the microcavity structure of the invention can be used, in particular, to produce security elements with high-resolution see-through motifs. The image information of the image is coded by the shape of the microcavity structure. A high degree of detail is possible without a complicated printing process, etc. would be required. The transmission of the microcavities is adjusted by the aspect ratio in combination with the coating of the microcavities. The aspect ratio of the microcavities is preferably between 0.3 and 0.8, more preferably between 0.3 and 0.5.

The optical properties of microcavities whose structure width is in the range of 5 μm and more and thus orders of magnitude greater than the wavelength of the incident light can be described by the beam optics. Such microcavities may have retroreflective properties for certain geometries. However, if the structure width is on the order of the wavelength of light, the light diffraction dominates the microcavities, whereas the laws of geometric optics are only valid as approximation (cf. H. Ichikawa, "Numerical analysis of microretroreflectors: transition from reflection to diffraction", J. Opt. A, Pure Appl. Opt. 6, p. 121, 2004 ). The diffraction behavior of microcavities with structure widths in the micrometer range, as used in the invention, has not yet been fully investigated scientifically. This applies in particular to microcavities which are coated with a multilayer structure.

The microcavities used in the security element are so large that light diffraction does not (anymore) dominate, but on the other hand so small that radiation optical reflection does not (yet) dominate. This intermediate area has been found to be advantageous for manufacturability and security against forgery of the security element. Below a dimension or aperture width of 2 to 3 .mu.m, diffraction of the light gradually occurs at structures in the order of magnitude of the wavelength, as a result of which the scattering or the diffraction fraction increases. Above this limit, on the other hand, the proportion of specular reflection increases. Then the profile shape of the microcavities becomes more relevant. A lateral feature size of a microcavity not exceeding 3 μm is therefore favorable. Below 0.5 microns dominated the light diffraction, which would make a simple imaging considerably more difficult. This results in a total of a lateral size of the microcavities in at least one spatial direction between 0.5 .mu.m and 3 .mu.m, more preferably greater than / equal to 0.7 microns and / or less than / equal to 2 microns.

The selected size range realizes a good optical effect of the microcavities with comparably simple production. Surprisingly, larger microcavities would increase the manufacturing effort, since then the exact profile shape would be relevant. The microcavities according to the invention are in particular no retroreflectors.

Each microcavity can be understood as a picture element in the form of a pixel producing a transparency in the form of a transparency. The selected size range leads to a good pixel density with usual image sizes.

It has been recognized by the inventor that the interaction of the aspect ratio of the microcavity structure and metal-containing coating in the aperture widths according to the invention makes it possible to see clearly in the image and also structured by the arrangement of the microcavities without, for example, structuring the coating being necessary.

A specific coating method, in particular a directional coating method is not required for this purpose. Thus, the coating thickness in each microcavity can be homogeneous and there is no thinning of the coating z. B. on the flanks of the microcavities needed. The layer thickness is thus optionally equal to the bottom and flanks of the microcavity. The production of the security element is thus simplified, and yet a fine-structured image is possible.

For the purposes of this description, a metal-containing coating is to be understood as meaning both a coating which consists only of metal and a layer structure which contains metal. Such a layer structure may consist of a single composite layer which has metal deposits. But it can also consist of a multi-layer structure in which one or more layers are metallic. An example explained in more detail below comprises an interference layer structure comprising a metal layer.

The invention further achieves that the security element can also be seen in reflection, since the metal-containing coating produces a higher reflection in the smooth areas than at the locations of the security element which carry microcavity structures. Since the microcavity structures increase the transparency of the see-through element, but reduce the reflection, an intensity inversion takes place between viewing in reflected light and in view. This increases the recognizability and formative effect of the security element.

The smooth areas may have some roughness unless an average aspect ratio of 0.2 is overlapped.

The microcavity structure allows to structure the image. In this case, for increasing distances between adjacent microcavities, the transmission is reduced in accordance with the area fraction of the microcavities with respect to the total area. A distance between two adjacent microcavities in the microcavity structure, which is between 0 and 200 nm, is therefore advantageous. According to a particularly preferred embodiment, the microcavities in the microcavity structure abut each other directly.

As already mentioned, the aspect ratio of the microcavities has an effect on their transmission behavior. It is therefore preferred to structure the first image area of the image, in particular the foreground, by two different types of microcavities which differ in aspect ratio. Deeper microcavities have higher brightness in transmitted light viewing than shallower microcavities.

A continuous intensity variation in transmitted light can be achieved if the aspect ratio of the microcavities in the microcavity structure varies according to a predetermined pattern defining the first image area, in particular the foreground of the image.

It is particularly preferred that the metal-containing coating is an interference layer structure with the layer sequence metal-dielectric-metal. Then, resonance effects can be exploited that play a role in certain aspect ratios, in particular in combination with specific aperture widths. In this way, not only the transmission and reflection effect of the micro cavity structures can be adjusted, it can also produce a color effect, which may depend for example on the viewing angle. In particular, microcavities coated with an interference layer structure show a strong intensity or color contrast with respect to the smooth area. As a dielectric, SiO ₂ comes into question, as metal aluminum or chromium.

A particularly good brightness difference in transmitted light is obtained in the case of an interference layer structure which is symmetrical with respect to its material sequence and / or layer thickness sequence. Particularly preferred is a layer sequence of aluminum-SiO ₂ -aluminum, wherein the layer thickness of Aluminum between 7 and 25 nm, preferably between 10 and 15 nm, and that of the SiO _{2 is} between 70 and 250 nm.

According to a further preferred embodiment, the metal-containing coating may be formed by a metal layer. Such a configuration is particularly advantageous if the smooth areas should appear opaque. The term metal also includes metal alloys. As metal layers, for example, layers of aluminum, copper, chromium, silver, gold or an Al-Cu alloy into consideration, wherein the layer thickness of the metal layer is preferably between 10 and 100 nm.

With certain embodiments of the metal-containing coating, namely with particularly high metal layer thicknesses, a particularly strong light-dark contrast in the transmitted light can be generated. However, this contrast comes at the expense of a color effect, which then can only be very weak. It is therefore particularly preferred for such embodiments that above or below the microcavity structure and the smooth area a color layer is provided in order to produce a color effect in the transmitted light.

The aspect ratio of the microcavities and the coating modulate the intensity with which the individual microcavities transmit light and thus make the image visible in transmitted light. The aspect ratio is the ratio of maximum depth to minimum lateral extent and varies according to the desired image structure in the security element. Elaborate printing processes to provide small yet finely structured images are therefore no longer necessary. Instead, simple replication methods can be used to create microcavities.

The image information can be represented by the corresponding distribution of the microcavities in the form of a monochrome image. Different aspect ratios or a continuous variation of the aspect ratio are of course possible and lead to a grayscale image.

From a manufacturing point of view, it is advantageous that the surface coating on the microcavities does not laterally change in the security element in order to generate different magnitudes in transmission. It is essentially the same for all microcavities, and yet a different transimpression of the microcavities with respect to the smooth area and also within the microcavity structure is achieved by microcavities with different aspect ratios.

The microcavities can basically have any shape in their opening, for example square apertures. However, it is also possible to design microcavities in such a way that their edges form the boundary region of a picture element or motif of the picture. As a result, image elements within the image can be sharply delimited from one another, since the edges of the image elements are very clearly visible.

The geometry of the microcavities is by no means limited to rotationally symmetrical shapes or shapes which have a circular outline in plan view, ie in their aperture. Aspheres or free-form surfaces, such as B. of lamp reflectors are known, can be used equally. For the microcavities, a non-rotationally symmetric aperture, in particular a channel-shaped aperture, is furthermore possible. The two types may then differ in the direction along which the non-rotationally symmetric microcavities extend. In particular, groove-shaped depressions may also be used which, for example, have the shape of a half-cylinder which is curved in one direction only, but is elongate in the other.

Furthermore, non-rotationally symmetric microcavities with different orientations can also be used to realize effects that convey a three-dimensional object impression to a viewer. For this purpose, the height information or the distance of the object to be displayed to the observer can be coded by the orientation angle of such microcavities. In this case, a viewer perceives a laterally different parallax in the flat, microcavity-structured surface. The spatial impression can be enhanced by additionally varying the structure depth of the microcavities as a function of the height or the reflection properties of the object during the production of the microcavities. A spatial impression can also be achieved if the intensity profile of the motif is converted pixel by pixel into such microcavities with encoded orientation angle.

For the production process according to the invention, in particular direct exposure techniques, for. B. with the help of a laserwriter in question. The preparation can be carried out analogously to the known production method for microlenses. The original of the microcavity structure is written via direct exposure with the help of a laserwriter in a photoresist coated substrate and then removed the exposed portion of the photoresist. An exposed original can then be galvanically formed and thus an embossing stamp can be created. Ultimately, the structure is produced by means of an embossing process, for example in UV varnish on film or directly (eg by hot embossing in the surface of the film). replicated. Alternatively, a nanoimprint method can be used. More elaborate methods of original manufacture, such as electron beam or focused ion beam imaging techniques, allow an even finer design of the geometry of the microcavities. These manufacturing methods offer many design options in the choice of the geometry of the microcavities. Thus, without additional effort, non-rotationally symmetric or non-spherical geometries of microcavities can be realized with higher accuracy than with the above-mentioned laser direct exposure method.

Subsequently, a coating of the surface, z. B. with a metal layer and / or an interference layer structure. Among others, electron beam vapor deposition, sputtering or thermal evaporation under vacuum are possible. Finally, the structure for protection is preferably laminated with a cover layer.

With constant aperture width, microcavities with different aspect ratios can also be generated solely by varying the depth of the microcavities, for example via a corresponding variation of the exposure intensity of a laserwriter. Such an approach offers the advantage of a much simpler data preparation.

The security element can in particular be designed as a security thread, tear-open thread, security tape, security strip, patch, film element or as a label. In particular, the security element can span transparent areas or recesses and / or be used for a security card or an identification document (eg passport).

In particular, the security element can be part of a precursor that can not yet be processed to a value document which, in addition to the security element according to the invention, can also have further authenticity features (such as, for example, luminescent substances provided in the volume). Under value documents here on the one hand the security element having documents understood. On the other hand, value documents can also be other documents and objects which can be provided with the security element according to the invention, so that the value documents have non-copyable authenticity features, whereby a verification of authenticity is possible and unwanted copies are prevented at the same time. Chip or security cards, such. Bank or credit cards are other examples of a value document.

The manufacturing method according to the invention can be designed so that the described preferred embodiments and embodiments of the security element are produced.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or alone, without departing from the scope of the present invention.

The invention will be explained in more detail by way of example with reference to the accompanying drawings, which also disclose features essential to the invention. For better clarity, a scale and proportioned representation is omitted in the figures. Show it:

1 a schematic representation of a part of a security element with a micro cavity structure,

2 a schematic representation to illustrate a coating of the micro cavity structure,

3 to 6 schematic representations of different geometries, which for the microcavity structure of 1 possible are,

7 to 9 Top views of a microimage, as in the embodiment according to 1 can be generated, and

10 and 11 schematic representations of two further embodiments.

In the figures below is a security element 1 described that provides an image in transmission, ie in transmitted light. The image is created by a microcavity structure.

Specifically, this is the security element 1 on a transparent foil 2 constructed on which an embossing lacquer layer 3 located. In the embossing lacquer layer 3 is a microcavity structure 4 molded, the microcavities 5 . 6 has different depth. Also is on the security element 1 a smooth area 7 available. As will be explained, the microcavities have expansions that can not be resolved by the unaided eye.

2 shows a schematic illustration of some microcavities. In the figure, a feature width w is entered. It represents in plan view the microcavity structure 4 the smallest cross section. Also in 2 the structure depth t of the individual microcavities is entered. The structure width w of each microcavity is between 0.5 and 3 μm. The structure depth t is chosen so that an aspect ratio t / w for each of the microcavities of the microcavity structure 4 greater than 0.3, preferably between 0.3 and 0.5, is given. The microcavities are so large that light diffraction does not (anymore) dominate, but on the other hand, in terms of their structure width w and the structure depth t, are so small that beam-optical effects do not (yet) dominate.

The microcavity structure 4 forms the security element 1 as a see-through element, wherein the brightness in transmission in the region of the microcavity structure 4 is different than in the smooth area 7 ,

1 shows schematically in the micro cavity structure 4 microcavities 5 and 6 different aspect ratio. For both microcavities 5 . 6 apply the size ranges mentioned, in particular an aspect ratio of 0.3 to 0.5. The smooth area 7 and the shallower microcavities produce less light transmission than the deeper microcavities.

However, this difference is not related to the fact that the embossing lacquer layer 5 in the area of the microcavity structure 4 would be thinner. Due to the layer thicknesses of the embossing lacquer layer, which are several orders of magnitude thicker than the structure depth t, and due to the fact that the embossing lacquer layer 3 is transparent, the different transmission in the transmitted light can not lack material thickness of the embossing lacquer layer 3 originate.

Also, the difference or the observed effect of the increased transmission in the areas provided with microcavities does not come about solely because relatively less metal accumulates in the microcavities because of the increased surface area of the microcavities compared to a flat area.

Rather, the different transmission is in particular caused by the fact that at the top of the security element 1 both in the area of the microcavity structure 4 as well as in the smooth area 7 a metal-containing coating 9 is provided. Without being bound by this explanation, an electromagnetic resonance effect presumably occurs due to the interaction of the aspect ratio of the microcavity structure and metal-containing coating at the aperture widths according to the invention, which additionally ensures that the incident radiation E is transmitted to a greater extent than transmitted radiation T, when the incident radiation E passes through the microcavity structure 4 falls. In the transmitted light view, the security element is therefore brighter at those points where the microcavity structure is located 4 located. Inside the microcavity structure 4 In this case, sections with deeper microcavities appear 6 even brighter than sections with comparatively shallower microcavities 5 ,

In a preferred embodiment, the coating is 9 constructed as interference layer system, for example in the form of a three-layer system. In one embodiment, the layer sequence Al-SiO ₂ -Al is used.

As a result, a structuring is created in the security element that provides image areas, namely image areas 8a passing through the smooth area 7 are formed and appear dark. Heller on the other hand are delineated image areas 8b and 8c in which the microcavity structure 4 lies, with the image area 8b appears brighter than the image area 8c because of its microcavities 5 have a lower aspect ratio than the microcavities 6 of the image area 8c ,

For the effect which, as explained below, is used for image formation, it is essential that the microcavity structure 4 like the smooth area 7 with the same coating 9 be provided. The brightness differences in transmission then require no structuring of the coating 9 , The image information is coded by an embossing process, namely by the appropriate embossing of the embossing lacquer layer 3 , A high-resolution printing process etc. is not necessary.

In one embodiment, it is provided that the thickness of the coating 9 also within each microcavity 5 . 6 is uniform. Then it is not necessary to apply special coating methods that lead to a thinning of the coating 9 in the flanks of the microcavities 5 . 6 the microcavity structure 4 lead, even if such coating methods according to the invention are of course possible.

In one embodiment of the invention, a coating method is used which comprises an undirected vapor deposition of the layers. Such a coating method is, for example, sputtering. In other embodiments, other layer application methods are used, for example, chemical deposition methods, etc., which provide a uniform layer thickness 9 - each seen to the surface normal - reach. With deposition method, a layer thickness is realized on the microcavity structure 4 and the smooth area 7 is essentially the same.

In a further embodiment it is provided that the thickness of the coating 9 on the flanks of the microcavities 5 . 6 the microcavity structure 4 thinned out. Such thinning on the flanks additionally increases the transmission in the region of the microcavities. Suitable coating methods that can be used for directional coating include electron beam evaporation or thermal evaporation.

The coating 9 is implemented in all embodiments, that the brightness between the smooth areas 7 and the microcavity structure 4 differs in transmission by at least 10%. By appropriate design of the coating 9 in combination with corresponding geometry of the microcavity structure 4 In this way it is ensured that in transmitted light a sufficient difference in brightness between the smooth areas 7 and the microcavity structure 4 is available.

The metallic coating 9 brings another advantage. In the overhead light, that is from the microcavity structure 4 generated motif also recognizable - with a contrast reversal compared to the transmitted light view.

Using the above-mentioned interference layer structure comprising aluminum and silicon in the layer sequence 15 nm Al / 90 nm SiO ₂ / Al 15 nm is obtained with an aspect ratio of 0.5 for the microcavities in transmission the following L * C * h-values: Area L * C * H 4 45 11 285 7 25 53 290

The brightness L * is thus within the range of the microcavity structure 4 almost twice as large as in the smooth area 7 , The chroma C *, however, is in the smooth range 7 significantly higher. The color h is largely similar for both areas.

In another exemplary embodiment, a security element has microcavities of different depths, namely comparatively flat microcavities 5 with an aspect ratio of 0.3 and comparatively deep microcavities 6 with an aspect ratio of 0.5. The element is vapor-deposited with an 80 nm-thick aluminum layer. The following L * C * h values are obtained: Area L * C * H 5 50 5 82 6 69 4 73 7 <1 - -

The brightness values L * show that the bright-dark contrast is very pronounced. The smooth area 7 is almost opaque. But it is also a clear contrast between deep and shallow microcavities 5 . 6 recognizable. The transmission is almost color neutral. It appears in a slight shade of green. The chroma values C * are relatively small. It is therefore provided in a development by applying a color layer above or below the embossing lacquer layer 3 additionally to produce a color in transmission.

The calculation of the values L *, C * and h of the above tables was based on the spectral emission of a D65 standard illumination and on the sensitivity curves of a 10 ° observer.

2 shows by way of example that the microcavity structure 4 and the smooth areas 7 with an additional topcoat layer 13 be covered and protected, for example, from contamination or impression by counterfeiters. The color layer mentioned for one embodiment may, for example, under the embossing lacquer layer 3 or over the topcoat layer 13 be attached.

The 3 to 6 show examples of different possible microcavities 4 , In 1 is an example of a microcavity structure 4 represented, whose surface is formed in each case by the geometry of a conic section. The apertures, ie the plan views along the optical axis, are equal in two lateral spatial directions, ie transverse to the optical axis. The apertures of the microcavities 3 are not limited to rotationally symmetric and not uniform forms for all microcavities. 3 shows a square aperture for the microcavities 5 which each have the same depth profile in sectional planes which are perpendicular to each other and intersect in the optical axis.

There are also recesses possible, which are curved in one direction and the other direction are not curved. 4 shows an embodiment with such a micro cavity structure 4 ,

The microcavity structures 4 of the 3 to 4 is common that their apertures, exemplified as a square or circle, have no structure width greater than 3 microns. For the generation of an image, however, it is sufficient if this limitation is observed only in one direction parallel to the surface. Corresponding non-rotationally symmetrical, channel-shaped microcavities 5 show the 5 , The microcavities 5 the microcavity structure shown there 4 extend along one direction 8th , possibly even over more than 3 microns.

The concave curvature of the microcavities 5 is not limited to semi-ellipses or semicircles. It can be used aspherical bulges or asymmetric forms, such as the 6 shows. Here are the microcavities 5 . although still concave, irregular curved surfaces and partly also convex sections.

The increase of light transmission at microcavities 5 Such micro cavity structures hardly depend on the detailed surface curvature since the minimum feature sizes are between 0.5 μm and 3 μm. Therefore, when interacting with the incident light, the laws of geometrical optics are no longer compulsory and only an interaction of electromagnetic waves takes place. Modifications or minor deviations in the curvature of microcavities in the wavelength or sub-wavelength range therefore have no significant effect on the transmission behavior of such microcavities 5 , This has the advantage that in the production of microcavity structure 4 the microcavities 5 , which should show different transmission, essentially only in one parameter, namely in the aspect ratio (maximum depth to minimum lateral extent) must be varied. This facilitates the production considerably, since no increased precision requirements are imposed on the profile shape.

7 shows a picture 11 with a motif 12 as number "25". It is according to microcavities 3 formed, so from micro cavities, which have a square aperture. Each microcavity forms a pixel 10 so that the micro cavity structure 4 a total of a first image area, in the embodiment, the foreground of the image 11 provides. In the microcavity structure 4 For example, the microcavities are periodically arranged in a fixed pattern. That is not mandatory; an aperiodic or quasi-statistical pattern is also possible. In the 7 Gray values displayed for the individual pixels reflect the aspect ratio of the individual microcavities. The microcavities within the motif 12 are about 20% deeper than the microcavities in the surrounding area. Around the microcavity structure 4 around is the smooth area 7 provided a second image area and here the background of the image 11 forms.

Microcavities, which are in the border area of the motive 12 lie, are optional on the side of the subject 12 lower than on the other side. This has the advantage that relatively fine patterns can be reproduced even with large pixelation or a small number of pixels due to the microcavities.

The design of an image 11 is not limited to microcavities with square or symmetrical apertures, such as 8th shows. Here's a picture 11 to see that a star as a motive 12 having. The edges of the apertures of the pixels 10 That is, the microcavities are designed to match the edge of the subject 12 coincide. This causes these edges to be reproduced very clearly by the image scattered at the boundary and the edge of the subject 12 especially good and sharp.

Also in the embodiment according to 8th is the microcavity structure 4 through the smooth area 7 surrounded, which here represents the image background. In the pictures of the 7 and 8th this background is designed as a comparatively narrow strip. This can only be seen as an example. Rather, the smooth areas 7 also larger surface sections of the security element 1 or the picture 11 cover. Also, it is possible that the smooth area 7 includes a demetallized portion for generating a further subject or pattern clearly visible in review such as reflection. Such demetallizations are also known in the art under the name "Cleartext".

The design of the motif 12 through the picture 11 is not on the choice between periodic pattern and aperiodic pattern for microcavity structure 4 limited. So it is possible the distance of the centers of the microcavities 5 , So the minimum feature size w, according to predetermined functions continuously along at least one direction to change.

The realization of an image 11 based on the microcavity structure 4 takes place in a particularly preferred embodiment such that the microcavity structure 4 represents a motif with its outline. An exemplary embodiment shows 9 , There is a bird's image through a microcavity structure 4 consisting of microcavities 6 greater depth and microcavities 5 shown smaller depth. 9 shows the brightness distribution in transmitted light. The microcavities 6 Due to their comparatively higher aspect ratio, they produce a higher intensity in transmitted light than the microcavities 5 , The presentation of the 9 is simplified to the effect that the microcavities themselves are not shown. The light gray areas are therefore of a variety of microcavities 6 formed, the dark gray areas of a variety of microcavities 5 , The dark sections of the picture 11 be through the background 7 educated.

In the picture 11 In addition, and by way of example, a highly transparent image element is present, which can be demetallised 14 in the smooth area 7 is produced. It is the already mentioned "Cleartext" element. The year figure in the upper part of the picture 11 was, however, by a structure of microcavities 6 generated. While the numerical representations show a strong contrast in reflection, both appear in Transmission bright in front of an opaque environment, although the through the microcavities 6 generated year figure representation is less bright than the demetallized area 14 ,

The highly transparent area 14 can be generated for example by laser irradiation metallized microcavities. By increasing the electromagnetic field in the microcavities, the laser light is preferably absorbed while it is reflected at a corresponding dosage in the region of the smooth surface. This results in laser ablation in the area of the microcavities. In this way, by means of fine structuring of the microcavities in interaction with a laser irradiation, highly transparent regions with a high resolution can be produced.

The production of the microcavity structure 4 is preferably as explained in the general part of the description, by laser direct exposure in a photolithographic process. microcavities 5 . 6 with dimensions down to a minimum diameter of 0.5 μm can be written directly into photoresist using a laserwriter. Due to the non-linear sensitivity of common photoresists, structures can be produced with a suitable choice of the exposure intensity, which are significantly finer than the beam diameter of the laser. The texture depth can be easily varied by choosing the exposure intensity.

For the production of microcavities with high accuracy, electron beam or focussed ion beam exposure methods may optionally be considered. An exposed original can then be galvanically molded after development of the photoresist and replicated via a stamping process in UV varnish on film or directly into the surface of the film. Alternatively, nanoimprint methods can also be used.

In a last step, the coating of the surface, z. B. by undirected steaming. Metallized surfaces or interference layer systems can be applied by electron beam evaporation, sputtering or by evaporation under vacuum. Subsequently, the coated side is preferred for protection with the topcoat layer 13 Mistake.

The pictures can also contain hidden information, eg. As micro-font, serial numbers, symbols, etc., which can not be resolved with an unaided eye, set down on a document of value. The micro images advantageously have much smaller structures than known laser engravings. The use of the microcavity structure 4 allows a very fine structuring of images or motifs with high lateral resolution, which is not possible with conventional printing methods.

10 shows an embodiment of the security element, in which the transmission effects of the invention microcavities are exploited for only to be considered in reflection color security element. The security element is under the embossing lacquer layer 3 a color layer 15 and below that a reflective layer 16 is provided. The reflective layer 16 may in particular be formed by a metal layer, for example by a layer of aluminum, copper, chromium, silver or gold. In this way, even with a metallic (and therefore possibly color almost neutral) coating 9 in the microcavity structure 4 and the smooth area 7 a color effect are generated. Optionally, the color layer 15 be formed translucent. The security element 1 contains (like the previously described security elements too) smooth areas 7 , which form image areas I and a microcavity structure 4 , which form an image area II. The coating 9 is designed in such a way that light is attenuated to an intensity of maximally 10% in the case of a two-time transmission through the image areas I, a twice transmission through the microcavity structure 4 in picture area II, however, is still ensured with sufficient intensity. This is the case, for example, with aluminum from a layer thickness of about 13 nm. In reflection, the image areas I and II differ by the optical effect of the micro cavity structure 4 , The incident light E falls on the underlying color layer 15 , Due to the metal layer provided there 16 it is mirrored and transmits again through the microcavity structure 4 (Arrow R). An observer therefore perceives this light path and thus the image area II in reflection in color, whereas the image areas I appear substantially unchanged metallic due to the low intensity of the transmitted light.

In an embodiment not shown here, instead of the topcoat layer 13 an embossing lacquer layer may be provided into which the microcavity structure 4 is molded. Instead of the embossing lacquer layer 3 out 10 can in this embodiment between the color layer 15 and the coating 9 For example, a further lacquer layer may be provided.

Leaving the reflective layer 16 As an alternative, an observer sees the transmitted radiation T in the region of the microcavity structure 4 , so in the image area II, lighter and with a through the color layer 15 given color.

In a further embodiment, the in 11 is shown and which also makes use of the transmission effects of inventive microcavities, is in addition to the color layer 15 another color layer 17 above the microcavity structure 4 and the smooth area 7 , This color layer 17 preferably has a different color than the color layer 15 , Under the paint layer 15 is also a reflective layer 16 For example, a layer of aluminum provided. The image areas I are accordingly perceived in reflection in the hue of the upper color layer, the image area II with the microcavity structure 4 however, as a mixed color of the two color layers 15 and 17 , Of course, in the embodiment of the 11 the reflective layer 16 omitted. Then the image area II appears in the mixed color passing through the color layers 15 and 17 is given, for the transmitted radiation T.

As already explained in the general part of the description, various modifications are possible within the scope of the claimed definition of the invention. Some are mentioned here by way of example:
The microcavity structure 4 has been described above in one embodiment, the microcavities 5 . 6 having different aspect ratio. This is not mandatory. It is both possible to use microcavities with a uniform aspect ratio, as well as aspect ratios that realize more than two different values.

The aspect ratio is set in the above embodiments by the variation of the texture depth t. Similarly, a variation of the feature size w is possible to modify the aspect ratio. Of course, both t and w can be varied within the given dimensions.

The smooth area 7 is entered in the drawings as completely smooth. This is not mandatory. It can also have a residual roughness, wherein an aspect ratio of 0.2 is not exceeded.

The orientation of the microcavity structure is irrelevant to the transmitted light effect. An inversion of the security element compared to the representation of the 1 respectively. 2 is possible at any time.

The microcavity structure and the smooth region for generating a transmitted light image can be combined in any manner with other security features known for the protection of value documents. This additional counterfeit security is achieved.

LIST OF REFERENCE NUMBERS

1

security element

2

foil

3

Embossing lacquer layer

4

Mikrokavitätenstruktur

5, 6

microcavities

7

smooth area

8a-c

image area

9

coating

10

pixel

11

image

12

motive

13

Topcoat

14

demetallization

15

coat of paint

16

reflective layer

17

coat of paint

t

structure depth

w

Forest-wide

e

incident radiation

R

reflected radiation

I, II

image area

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1434695 B1 [0005]
• WO 2005/106601 A2 [0005]
• EP 1979768 A1 [0005]
• WO 2002/101669 A2 [0005]
• EP 1476317 A1 [0005]
• US 7468842 B2 [0005]
• DE 102008046128 A1 [0006]
• WO 2005/095119 A1 [0007]
• DE 102006050047 A1 [0008]
• EP 1786632 B1 [0009]
• WO 2012/069163 A1 [0010]

Cited non-patent literature

• H. Ichikawa, "Numerical analysis of microretroreflectors: transition from reflection to diffraction", J. Opt. A, Pure Appl. Opt. 6, p. 121, 2004 [0016]

Claims (23)

Security element for the production of value documents, such as banknotes, checks or the like, comprising a substrate ( 3 ) with an upper side and at least one image ( 11 ), where - the image ( 11 ) through a microcavity structure ( 4 ), which have a plurality of juxtaposed microcavities ( 5 . 6 ), and a smooth area ( 7 ), - the microcavities ( 4 ) each have a feature size (w) of 0.5 μm to 3 μm and perpendicular thereto a structure depth (t) in a spatial direction lying parallel to the top side and have an aspect ratio which is defined by the ratio of feature depth (t) to feature size (w) is defined, and - the microcavity structure ( 4 ) and the smooth area ( 7 ) a metal-containing coating is applied, characterized in that - the aspect ratio of the microcavities ( 5 . 6 ) is greater than 0.3 and - the metal-containing coating ( 9 ) on the microcavity structure ( 4 ) is designed such that the microcavity structure ( 4 ) and the smooth area ( 7 ) have a brightness difference of at least 10% in transmitted light observation, so that the microcavity structure ( 4 ) a first image area and the smooth area ( 7 ) a second image area of the image ( 11 ) form. Security element according to claim 1, characterized in that the aspect ratio of the microcavities ( 5 . 6 ) is between 0.3 and 0.8, preferably between 0.3 and 0.5. Security element according to claim 1 or 2, characterized in that the metal-containing coating ( 9 ) on the microcavity structure ( 4 ) has the same layer thickness. Security element according to one of claims 1 to 3, characterized in that in the microcavity structure ( 4 ) the microcavities ( 5 . 6 ) abut each other directly. Security element according to one of claims 1 to 4, characterized in that the microcavity structure ( 4 ) at least two types of microcavities ( 5 . 6 ), which differ in terms of the aspect ratio, the first image area of the image ( 11 ) by the at least two different types of microcavities ( 5 . 6 ) is structured. Security element according to one of claims 1 to 4, characterized in that the aspect ratio of the microcavities ( 5 . 6 ) in the microcavity structure ( 4 ) according to a first image area of the image ( 11 ), predetermined pattern varies. Security element according to one of claims 1 to 6, characterized in that the metal-containing coating ( 9 ) is an interference layer structure with the layer sequence metal-dielectric-metal. A security element according to claim 7, characterized in that the dielectric is SiO ₂ and / or the metal is Al or Cr. Security element according to claim 7 or 8, characterized in that the interference layer structure is symmetrical with respect to its material sequence and / or layer thickness sequence, in particular the layer sequence has Al-SiO ₂ -Al. Security element according to one of claims 1 to 6, characterized in that the metal-containing coating ( 9 ) is formed by a metal layer. Security element according to one of claims 1 to 10, characterized in that above or below the microcavity structure ( 4 ) A color layer is provided. Security element according to one of claims 1 to 11, characterized in that the microcavities ( 5 . 6 ) are non-rotationally symmetrical, in particular channel-shaped, and each extend along one direction. Value document with a security element ( 1 ) according to one of claims 1 to 12. Manufacturing method for a security element ( 1 ) for value documents, such as banknotes, checks or the like, the at least one image ( 11 ), wherein - on a substrate ( 3 ) having an upper side for generating an image ( 11 ) a microcavity structure ( 4 ), which have a plurality of juxtaposed microcavities ( 5 . 6 ), and a smooth area ( 7 ), - the microcavities ( 5 . 6 ) are each formed in a spatial direction lying parallel to the upper side with a structure width (w) of 0.5 μm to 3 μm and perpendicular to it with a structure depth (t) and have an aspect ratio which is defined by the ratio of structure depth (t) to structure width (w) is defined, and - the microcavity structure ( 4 ) and the smooth area ( 7 ) a metal-containing coating ( 9 ), characterized in that - the aspect ratio of the microcavities ( 5 . 6 ) is greater than 0.3 and The metal-containing coating ( 9 ) on the microcavity structure ( 4 ) is formed so that the microcavity structure ( 4 ) and the smooth area ( 7 ) have a brightness difference of at least 10% in transmitted light observation, so that the microcavity structure ( 4 ) a first image area and the smooth area ( 7 ) a second image area of the image ( 11 ) form. Manufacturing method according to claim 14, characterized in that the aspect ratio of the microcavities ( 5 . 6 ) is between 0.3 and 0.8, preferably between 0.3 and 0.5. Manufacturing method according to claim 14 or 15, characterized in that the metal-containing coating ( 9 ) on the microcavity structure ( 4 ) has the same layer thickness. Production process according to one of Claims 14 to 16, characterized in that in the microcavity structure ( 4 ) the microcavities ( 5 . 6 ) are formed directly abutting each other. Manufacturing method according to one of Claims 14 to 17, characterized in that the microcavity structure ( 4 ) with at least two types of microcavities ( 5 . 6 ), which differ in terms of the aspect ratio, the first image area of the image ( 11 ) by the at least two different types of microcavities ( 5 . 6 ) is structured. Manufacturing method according to one of claims 14 to 17, characterized in that the aspect ratio of the microcavities ( 5 . 6 ) in the microcavity structure ( 4 ) according to a first image area of the image ( 11 ), predetermined pattern is varied. Manufacturing method according to one of claims 14 to 19, characterized in that as coating ( 9 ) an interference layer structure with the layer sequence metal-dielectric-metal is used. Manufacturing method according to claim 20, characterized in that as a dielectric SiO ₂ and / or as metal Al or Cr is used. Manufacturing method according to one of claims 14 to 21, characterized in that above or below the microcavity structure ( 4 ) A color layer is provided. Manufacturing method according to one of Claims 14 to 22, characterized in that the microcavities ( 5 . 6 ) are non-rotationally symmetrical, in particular groove-shaped, and in each case along one direction ( 8th ).

Images