EP3184319A1 - Safety element for security papers, valuable documents or the like - Google Patents

Abstract

The invention relates to a security element (12) for an object to be protected, such as. A security paper, document of value or the like comprising a substrate (14) having a plurality of microreflectors (22) arranged in a pattern and a plurality of microstructures which together with the microreflectors (22) produce an image perceivable by a viewer, each microstructure formed as a sub-wavelength grating (20) and associated with one of the microreflectors (22), whereby grating reflectors (10) each comprising a micro-reflector (22) and at least one sub-wavelength grating (20) are formed, each sub-wavelength grating (20) is formed so that it visible radiation incident through an aperture (24) of the microreflector (22) from a hemisphere, diffracts into a zeroth order of diffraction in reflection.

Description

The invention relates to a security element for an object to be protected, such as e.g. a security paper, document of value or the like comprising a plurality of microreflectors arranged in a pattern and a plurality of microstructures which together with the microreflectors produce an image perceptible by a viewer.

The invention further relates to a security paper or document of value.

Finally, the invention also relates to a method for producing a security element for an object to be protected, such. A security paper, document of value or the like, wherein on a substrate a plurality of microreflectors arranged in a pattern and a plurality of microstructures are formed, which together with the microreflectors produce an image perceptible by a viewer.

Items to be protected are often provided with a security element that allows verification of the authenticity of the item and thus serves as protection against unauthorized reproduction. Such items are, for example, security papers, identification or value documents (such as banknotes, chip cards, passports, identification cards, identity cards, shares, bonds, certificates, vouchers, checks, tickets, credit cards, health cards) as well as product security elements such. Labels, seals and packaging. It can also be products themselves, such as a capsule of a drug for which fakes are to be feared.

For security elements, so-called moiré magnification arrangements are described extensively in the prior art, for example in the documents EP 1434695 B1 . WO 2005/106601 A2 . EP 1979768 A1 . EP 1182054 B1 . WO 2011/029602 A2 . WO 2002/101669 A2 . EP 1476317 A1 and EP 1893074 A2 , These magnification arrangements combine focusing elements with microimages, which are located in the image plane of the focusing elements. The microimages are aligned with the focusing elements in such a way that a synthetic image results when the security element is viewed through the so-called moiré effect. This synthetic image has properties (for example, an ortho-parallactic effect) that are not reproducible by simply copying the images. The focusing elements can be designed as microlenses or microreflectors. The latter construction is the subject of the publications WO 2010/136339 A2 and the WO 2011/012460 A2 ,

It is common for known moiré magnification arrangements that the microimages are a greatly reduced form of at least a partial section of the synthetic image. They are formed for example by the image content corresponding relief surfaces which are filled with color, or which otherwise have light-absorbing properties.

The known security elements require a distance between microimages and focusing elements, which corresponds approximately to the focal length of the focusing elements. In the prior art, this requirement is generally met by the fact that microimages and microfocusing elements are arranged on opposite sides of a film whose thickness corresponds approximately to the focal length of the focusing elements. This procedure requires a double-sided embossing of the film in very exact register each other. This is expensive and therefore disadvantageous.

The generic document WO 2014/012667 A1 also deals with a security element. It consists of a large number of microreflectors arranged in a pattern, at the bottom of each of which there is a reflective grating, which diffracts incident radiation and directs it towards the microreflector, which then reflects it back to the viewer. Thus, incident light from the array of micromirrors and gratings is completely reflected to the viewer. The grids bend the incident light in the first diffraction order. By varying the relative position of microreflector to grating in the pattern, an image can be generated.

Proceeding from this, the object of the invention is to achieve greater protection against forgery and applicability in the case of a security element, a security paper or document of value as well as in a production method for these objects.

This object is solved by the features of the independent claims. The dependent claims relate to preferred embodiments of the invention.

The invention provides a security element for an article to be protected, such as a security paper, a document of value or the like. The security element comprises a substrate having a plurality of microreflectors arranged in a pattern and a plurality of microstructures which, together with the microreflectors, produce an image perceptible by a viewer. Each microstructure is designed as a sub-wavelength grating and assigned to one of the microreflectors, whereby grating reflectors comprising in each case a microreflector and at least one sub-wavelength grating are formed. Each subwavelength grid is designed to that visible radiation, ie light incident through an aperture of the micro-reflector from the half-space, diffractively diffracts into a zeroth reflection diffraction order, wherein preferably for each grating reflector, the sub-wavelength grating and the micro-reflector are coordinated so that the micro-reflector at least a portion of radiation diffracted by the sub-wavelength grating into the zeroth reflection diffraction order is reflected back into the half-space as re-radiation.

In a preferred embodiment, the sub-wavelength grating is formed as a semi-transparent sub-wavelength grating which transmissively diffracts a second portion of the visible radiation incident through the aperture of the microreflector from the half space into a zeroth transmission diffraction order. Preferably, in each grating reflector the subwavelength grating and the microreflector are matched to one another in such a way that the subwavelength grating transmits the radiation incident through the aperture of the microreflector from the half space and reflected by the microreflector in the zeroth transmission diffraction order through the substrate.

According to another preferred embodiment, the sub-wavelength grating is formed as an opaque sub-wavelength grating.

The subwavelength gratings are configured with structures whose period is smaller than the wavelength of the visible light spectrum. They are further designed so that they bend the majority of radiation coloring in the zeroth order of diffraction. If higher diffraction orders occur for certain angles of incidence in the short-wave wavelength range, only a small portion of the incident radiation is diffracted into these orders. For example, get 50%, 60% or 70% of the incident radiation in the zeroth order of diffraction, either in reflection or in transmission. In particular, the subwavelength grating can be designed in such a way that approximately half of the radiation is reflected and the other half is transmitted, and furthermore a small proportion of the radiation is absorbed. Other divisions are possible. The sub-wavelength grating is semi-transparent in this sense and causes a color in reflection and transmission. For certain parameters, the proportion of radiation that is absorbed may also be dominant.

The reflection and transmission properties of the subwavelength grating are particularly dependent on the wavelength of the incident radiation. In particular, only radiation in a specific wavelength range in the zeroth diffraction order is diffracted or reflected diffracted. Thus, it is possible to determine the color or the color appearance of the image generated by the grating reflector in reflection and / or transmission. In addition, the reflection and transmission properties of the subwavelength grating also depend on the profile geometry and on the material properties of the subwavelength grating.

The transmission takes place through the substrate, which is therefore preferably at least partially transparent or translucent for the radiation. The substrate is preferably made transparent at least in the region of a bottom of the grating reflectors, in particular the substrate is completely transparent, for example a transparent film. Optionally, outside of the grating reflectors, the substrate may be opaque, for example, coated with a sub-wavelength absorbing grating to provide a black background for the image.

The subwavelength grating and the microreflector are in particular arranged such that the radiation transmitted or reflected by the first or higher diffraction order is not reflected or transmitted as far as possible to the microreflector or at such shallow angles that the microreflector rejects radiation from the higher diffraction orders compared to the one in the zeroth Diffraction order transmitted only to a lower intensity component or reflected in the half space.

The arrangement of the subwavelength grating and the microreflector takes place in comparison to the approach of WO 2014/012667 A1 not with regard to the reflection in the first diffraction order, but in the zeroth order of diffraction. In addition, the security element is characterized in particular by the fact that the security element displays an image or several images to an observer, both when it is in the same half-space as the security element illumination (reflected image or supervisory image) and when the illumination of the security device is illuminated Security element and the observer with respect to the security element are located in two different half-spaces (transmitted image or transparency). The security element thus shows an image from both sides, the images being the same or different, e.g. B. with regard to a color impression can be.

The sub-wavelength grating of the security element is not used in advantageous embodiments for the absorption of radiation, but for the reflection and transmission of radiation. In particular, the part of the radiation absorbed by the sub-wavelength grating and / or the security element is extremely small, for example less than 50% of the incident radiation. In general, the absorption ratio is greater for a sub-wavelength grating than for a first-order diffraction grating, since electromagnetic resonances occur here for specific wavelengths which lead to light absorption, as for example in the documents DE 10 2011101 635 A1 and DE 10 2009 056 933 A1 described, the disclosure of which is included in the present application in this respect.

In further advantageous embodiments, the sub-wavelength grating is used only for the reflection of radiation and acts as a so-called subtraction color filter. The (opaque) sub-wavelength grating absorbs radiation in a certain wavelength range in the visible part of the radiation spectrum, the remaining part of the radiation spectrum is reflected. Thus, for example, if the blue portion of the radiation is absorbed, the subwavelength grating appears in reflection in the complementary color of blue.

The diffracted back and transmitted transmission radiation may optionally be mirrored prior to diffraction (transmission or transmission) or after or before diffraction (reflection) at the micro-reflector. In all cases, the direction in which it is emitted, the angle of radiation and the color of the formation of the grating reflector depends. By arranging differently configured grating reflectors in the pattern, colored symbols or images can be generated, both in a plan view and in a transparent view.

The portion of the incident radiation deflected by the grating reflector is responsible for producing the reflected (top view) or transmitted (see through) image. Due to the design of the grating reflector such that as much light is deflected, a particularly light-intensive image can be generated. The degree of deflection depends inter alia on the diameter of the aperture. The larger the diameter of the Aperture, the smaller the angular range in which the incident light is deflected by the grating reflector. On the other hand, as the diameter of the aperture increases, the convergence of the laterally deflected light increases. Thus, although a larger diameter lattice reflector of the aperture is capable of deflecting light from a wider range of angles, the divergence of the deflected light is also greater, which reduces the intensity of the image.

On the other hand, the size of the deflection also depends on the depth of the grating reflector, which is measured from the bottom of the grating reflector to its aperture. The deeper the grating reflector is, the lower the acceptance angle of the incident light, i. Only in a special, decreasing angle range is light deflected laterally by the grating reflector, so that a visible image is formed. However, as the depth of the grating reflector increases, so does the divergence of the reflected or transmitted light.

By a suitable choice of the diameter of the aperture and the depth of the grating reflector thus the acceptance angle of the incident light and the divergence of the reflected light or transmitted light can be selected accordingly to provide for the respective security element best parameters for generating the image.

A viewer perceives from a viewing direction a single grating reflector with an intensity and / or color determined by its design, but without image information. The image information is only achieved by the interaction of the grating reflectors in the pattern. Preference is given to multiple grating reflectors with different configurations (combination of microreflector and microstructure in the respective grating reflector) provided so that there is a difference in color or intensity between the grating reflectors, which produces a total of the perceptible image. In addition, a plurality of equal grating reflectors can be arranged side by side, with their arrangement carrying the image information. In this way, for example, letters or symbols are displayed. The image information is preferably generated by the security element in that each grating reflector consisting of microreflector and subwavelength grating has the function of a pixel for image generation.

Since the sublength wave gratings in combination with the microreflectors generally have strongly angle-dependent properties, it is also possible to generate parallax images as in a moiré magnification arrangement. It is also possible to form stereograms, for example by appropriate alignment of the microreflectors so that some grating reflectors provide image information for the left eye, others for the right eye (corresponding to the visual angle difference).

Finally, it should be noted that the above-described grating reflectors can be combined with known structures such as holograms, micromirror arrangements and / or light-absorbing structures such as moth-eye structures or microcavities.

The security element can be produced in a single molding process. It is not necessary to carry out registration steps to be carried out in register with one another on different sides of a transparent film. Rather, it is possible with a single embossing process, a substrate, for. As a film to be designed so that both the structures of the microreflectors and the sub-wavelength grating are generated. The relative position (and shape) of subwavelength grating and associated microreflector is determined by the given embossing tool, so that no mutually in the register standing sequences of processing steps on the substrate are necessary. This simplifies the production.

Furthermore, the minimum thickness of the security element is not predetermined by a focal length of focusing elements. The thickness is limited only by the depth of the microreflectors. This depth corresponds approximately to the height which microlenses would have for known moiré magnification arrangements, with the result that the minimum thickness of the security element is only a fraction of that of conventional security elements with moiré magnification arrangements. Nevertheless, a moiré effect can be realized equally.

The micro-reflectors can be configured as channel-shaped reflectors. They are then gutters, which preferably have a flat bottom. The flat floor does not necessarily have to be reflective. At the level bottom of the sub-wavelength grating is arranged, which is then also designed as a linear grating. Especially with channel-shaped reflectors, it is also possible to arrange more than one sub-wavelength grating along the channels of the reflector. Optionally, the micro-reflectors may be designed as a concave mirror, which preferably have a flat bottom on which the sub-wavelength grating is located. Again, the floor does not have to be reflective. The planar bottom of the microreflectors can in particular be configured obliquely, so that the sub-wavelength grating tends toward the microreflector.

Alternatively, it is also possible to use microreflectors whose bottom is not flat but, for example, arched.

To use the individual grating reflectors as pixels, is particularly simple if the micro-reflectors are formed as a concave mirror, which are rotationally symmetric, for example, have the shape of the lateral surface of a spherical layer or ellipsoid layer. Such a lateral surface is obtained when a layer is cut out of an ellipsoid or a sphere by two parallel planes which actually cut the sphere or the ellipsoid. The smaller of the two parallel circular surfaces or ellipses created by the cut then represents the bottom of the concave mirror. This does not have to be mirrored.

It is preferred that the microreflectors have, in addition to the bottom, a side wall which is parabolic in cross-section, straight, elliptical or formed as a hybrid form thereof.

The sidewall may be divided into several sections, each section being parabolic, straight, elliptical, or a hybrid form thereof. It is also possible that the individual sections are formed differently. If an aperture of hexagonal cross-section is provided, each section of a wall may correspond to the hexagonal aperture. Alternatively, the aperture can be rectangular and thus the micro-reflector cuboid, wherein each side wall in cross-section corresponding parabolic, straight, elliptical or formed as a hybrid form thereof. The shape of the side wall in cross section depends in particular on the size of the microreflector and the arrangement of the subwavelength grating in the microreflector.

The alignment of the side walls of the individual grid reflectors makes it possible to transmit image information. For example, in a first group of grating reflectors, the sidewalls are oriented in a first direction, so that these grating reflectors are visible in a first viewing direction. In a second group of grating reflectors, the side walls are aligned in a second direction, so that these grating reflectors can be seen in a second viewing direction.

It is preferred that the micro-deflectors are asymmetrical in cross-section, wherein in particular a first part of the side wall is straight and a second part of the side wall is concave, in particular parabolic or elliptical.

Preferably, the first part of the side wall and the second part of the side wall are arranged opposite each other. For example, in the case of a cuboid microreflector, two sections of the side wall are located opposite one another. By using asymmetric reflectors, the amount of light that is deflected laterally can be increased. Thus, the contrast of the image resulting from the pattern can be increased.

Further, the first part of the side wall, which is currently formed, may be arranged perpendicular to the ground. As a result, it can be achieved in particular that the straight part shields a region of the bottom from the incident radiation, i. shaded, and thus does not contribute to the undeflected portion of the incident radiation. Consequently, the proportion of radiation that is deflected laterally increases.

For common optical designs, it is preferred that the microreflectors have a depth of 2 to 30 microns, preferably from 5 to 20 microns. Further, it is preferable that each sub-wavelength grating has a grating period between 100 nm and 500 nm, particularly 240 nm to 420 nm, the sub-wavelength grating being one-dimensionally periodic or even two-dimensional can be formed periodically. Such structural dimensions give good results with at the same time easy manufacturability. The grating period and the profile of the subwavelength grating depend in particular on the color which is to be reflected or transmitted in the zeroth diffraction order.

On its side facing the half-space, the microreflectors and the sub-wavelength gratings are advantageously coated with at least one metallic layer, preferably with Al, Ag, Au, Cu, Cr or an alloy containing these metals.

If one wishes to increase the chroma in reflection, the microreflectors and the subwavelength gratings can be provided on their side facing the half space with a multilayer coating, e.g. B. as a trilayer of two superimposed metal or semiconductor layers with an intervening dielectric layer can be constructed.

Due to the shape of the aperture, the shape of the microreflector is determined in particular. Thus, the arrangements of the side surfaces are given and consequently also the reflected radiation from the side walls. For example, by using a rotationally symmetrical microreflector with a circular aperture, an image can be generated in many directions. When using a cuboid microreflector with a rectangular aperture individual observation directions may be preferred. The same applies to a cylindrical microreflector with a hexagonal aperture.

Grid reflectors with round apertures are preferably arranged in a hexagonal pattern, since in this way the highest possible area filling can be achieved.

The individual grid reflectors can be formed with webs surrounding them, i. H. at least some adjacent grille reflectors then do not abut each other directly, but are separated by a web. Between the grid reflectors are thus web surfaces, which are preferably formed by flat surfaces. By coating the web surfaces and / or shaping the thickness of the substrate in the region of the web surfaces, it is possible to ensure that light incident on the web surfaces is differently reflected and / or transmitted. The effect is preferably designed to be laterally variable in order additionally to encode symbols and make them visible in transmission. This additional counterfeit security is achieved.

A laterally varying coating of the web surfaces is preferably realized in that the web surfaces are first provided with a coating, for example with a metallization, and this is partially removed again, for. B. by an etching process. Alternatively, it is possible to partially transfer the coating present on the web surfaces by laminating with an acceptor film and thus to remove it from the web surfaces in certain areas. Further details of such a metal transfer process can the document WO 2011/138039 A1 which in this respect is fully incorporated into the disclosure of this description.

The micro-reflectors can basically have any shape in their aperture (opening), for example square, circular or rectangular apertures.

It is preferred that the aperture has a diameter and the microreflector has a depth from the aperture to the bottom, wherein within a pattern at least one of the following properties of the grating reflectors varies to produce an image: location of the subwavelength gratings to the respective associated microreflector The diameter of the aperture, the depth of the microreflector, the orientation of the microreflector, the shape of the microreflector and / or the grating period of the subwavelength grating.

By this variable arrangement special types of image can be generated. In particular, it is thus possible to generate images due to a moiré effect, a stereoscopic effect and / or a running effect.

In particular, one or more parameters are changed between the individual grating reflectors of the pattern. For example, a region of the image can be generated by grating reflectors with a first parameter set, while a second region of the image is generated by grating reflectors with a second parameter set. In this way, for example, generate stereograms.

Alternatively, the parameters within the image may change continuously to produce moiré effects, for example.

The security element may in particular be designed as a security thread, tear-open thread, security strip, security strip, patch or label. In particular, the security element can cover transparent areas or recesses of an object to be protected.

In particular, the security element can be part of a precursor that can not yet be processed to a value document, which, for example, can also have additional authenticity features (such as, for example, luminescent substances provided in the volume, etc.). Under value documents here on the one hand understood the document having security element, on the other hand value documents can also be other documents or items that can be provided with the security element according to the invention, so they have non-copyable authenticity features. Chip or security cards, such. As bank or credit cards are other examples of value documents.

Furthermore, the invention relates to a method for producing a security element for an object to be protected, such as. A security paper, document of value or the like, wherein on a substrate a plurality of microreflectors arranged in a pattern and a plurality of microstructures are formed, which together with the microreflectors produce an image perceptible by a viewer. Each microstructure is formed as a sub-wavelength grating and associated with one of the microreflectors, thereby forming grating reflectors each comprising a microreflector and at least one sub-wavelength grating. Each sub-wavelength grating is formed to reflectively deflect visible radiation incident through an aperture of the micro-reflector from a hemisphere into a zeroth reflection diffraction order, preferably matching the sub-wavelength grating and the microreflector for each grating reflector such that the microreflector matches Reflected radiation from the sub-wavelength grating in the zeroth reflection diffraction order reflected back radiation in the half space back.

According to an advantageous variant of the method, the sub-wavelength grating is formed as a semi-transparent sub-wavelength grating which transmissively diffracts a second portion of the visible radiation incident through the aperture (24) of the microreflector (22) from the hemisphere into a zeroth transmission diffraction order. In each lattice reflector, the sub-wavelength grating and the microreflector are preferably further tuned to one another such that the sub-wavelength grating transmits the radiation incident through the aperture of the microreflector from the half-space and reflected by the microreflector in the zeroth transmission order.

According to a further advantageous variant of the method, the sub-wavelength grating is formed as an opaque sub-wavelength grating.

For the production method according to the invention, in particular direct exposure techniques are suitable for producing the microreflectors, e.g. B. photolithographically with the help of a laserwriter. The preparation can be carried out analogously to the known production method for microlenses. Regardless, the structure of the subwavelength gratings is generated. The two processes are preferably carried out accurately in one and the same photoresist. Alternatively, two different Belackungsvorgänge are possible. After removal of the exposed portion of the photoresist, an exposed original can then be galvanically molded, thus producing an embossing stamp. Ultimately, the structure is replicated via a stamping process, for example in UV varnish on film. Alternatively, a nanoimprint method can be used. More elaborate methods of original manufacture, such as electron beam or focused ion beam exposure techniques, allow for even finer geometry design and are therefore particularly suitable for the production of subwavelength gratings.

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.

Fig. 1

in a) und b) Gitterreflektoren mit jeweils symmetrischen Mikroreflektoren und unterschiedlichen Tiefen;

Fig. 2

in a) und b) Gitterreflektoren gemäß einer zweiten Ausführungsform mit asymmetrischen Mikroreflektoren und unterschiedlicher Tiefe;

Fig. 3

in a) bis c) den Gitterreflektor aus Fig. 1 mit jeweils unterschiedlicher Tiefe;

Fig. 4

in a) bis c) den Gitterreflektor aus Fig. 1 mit jeweils unterschiedlichem Durchmesser einer Apertur;

Fig. 5

in a) und b) zwei verschiedene Typen von Gitterreflektoren mit konischer und Parabel-Querschnittsform und unterschiedlicher Tiefe;

Fig. 6

in a) und b) verschiedene Typen von Gitterreflektoren mit asymmetrischen Mikroreflektoren mit Parabel-Form und konischer Form sowie unterschiedlicher Tiefe;

Fig. 7

in a) bis c) drei verschiedene Ausführungsformen von Sicherheitselementen, bei welchen sich die Lage eines Subwellenlängengitters hinsichtlich des Mikroreflektors über das Sicherheitselement ändert;

Fig. 8

ein Sicherheitselement aus einer Anordnung von Mikroreflektoren mit Subwellenlängengittern, bei denen die Mikroreflektoren je nach Gruppe unterschiedlich orientiert sind; und

Fig. 9

die Entstehung des durch das Sicherheitselement hervorrufbaren Bildes.

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:

Fig. 1

in a) and b) grating reflectors, each with symmetrical micro-reflectors and different depths;

Fig. 2

in a) and b) grating reflectors according to a second embodiment with asymmetrical microreflectors and different depths;

Fig. 3

in a) to c) the lattice reflector Fig. 1 each with different depths;

Fig. 4

in a) to c) the lattice reflector Fig. 1 each with a different diameter of an aperture;

Fig. 5

in a) and b) two different types of grating reflectors with conical and parabolic cross-sectional shape and depth;

Fig. 6

in a) and b) various types of grating reflectors with asymmetrical microreflectors with parabolic shape and conical shape and depth;

Fig. 7

in a) to c) three different embodiments of security elements, in which the position of a sub-wavelength grating with respect to the micro-reflector changes via the security element;

Fig. 8

a security element of an array of microreflectors with subwavelength gratings, in which the microreflectors are oriented differently depending on the group; and

Fig. 9

the emergence of the image elicited by the security element.

The Fig. 1 to 9 The safety element 12 is made of a substrate 14, for example a transparent film, on whose upper side (the term is to be understood purely by way of example and is not intended to indicate a preferred direction) an embossed structure is formed. The embossing structure comprises a plurality of microreflectors 22, which in the embodiments of the Fig. 1 to 4 are formed as elliptical reflectors with a flat bottom 16. In other embodiments, for example, in which the Fig. 5 and 6 the micro-reflectors are designed in other geometries, in particular not rotationally symmetrical.

Each grid reflector 10 thus has the bottom 16 and at least one side wall 18. A semi-transparent sub-wavelength grating 20 is provided on the bottom 16 of the grating reflector 10. The at least one side wall 18 and the bottom 16 form the microreflector 22, which is open to the top and thus to a half space through an aperture 24 with a diameter a. The distance from the bottom 16 to the aperture 24 determines the depth t of the grating reflector 10. The substrate 14 is at least partially transparent in the region of the bottom 16.

The side wall 18 of Fig. 1 illustrated grid reflectors 10 is designed parabolic in cross section. Furthermore, here the grating reflector 10 is designed rotationally symmetrical. The grating reflectors 10, which are in Fig. 1a and 1b are shown to have the same diameter a of the aperture 24, but differ in their depth t. The ratio of the diameter a of the aperture 24 to the depth t of the grating reflector 10 is 0.8 in Fig. 1a and 0.55 in Fig. 1b ,

The proportion of the radiation incident from the half-space, which falls on the sub-wavelength grating 20, is reflected directly there and transmitted to a second part. The other remainder of the visible radiation incident to be reflected by the micro-reflector 22 onto the sub-wavelength grating 20 is also partially diffracted by the sub-wavelength grating 20 in zero diffraction order and partially transmitted. Reflection in the zeroth reflection diffraction order means that the angle of incidence is equal to the angle of reflection, but z. B. the color properties of the diffraction are dependent on the direction of the incident light. The proportion of light which is first reflected at the micro-reflector 22 is thus reflected differently at the sub-wavelength grating and transmitted as the directly incident light component. The incident radiation is deflected laterally with respect to the angle of incidence, so that it creates a visible image for the observer.

The amount of light deflected laterally can be increased by providing an asymmetrical grating reflector 10 as shown in FIG Fig. 2a and 2b is shown. A first part 26 of the side wall 18 is straight in cross-section and is in particular perpendicular to the bottom 16. A second part 28 of the side wall 18, which is arranged opposite the first part 26, is as in FIG Fig. 1 parabolic shaped. The ratio of the diameter a of the aperture 24 to the depth t of the grating reflector 10 is 1 in Fig. 2a and 0.53 in Fig. 2b ,

Like this in Fig. 2a and 2b is represented by the left light beam, the first portion 26 of the side wall 18 shadows a portion of the sub-wavelength grating 20 in the incident radiation. This increases the proportion of the light which is deflected by the grating reflector 10. The smaller the depth t of the grating reflector 10, the lower the shaded area, so that the deflected radiation component decreases with decreasing depth t.

Fig. 3 shows three different depth grating reflectors 10 with a constant diameter a of the aperture 24 and the same sub-wavelength grating 20 at the bottom 16 of the grating reflector 10. This figure can be assigned the following dimensionless values: a = 0.4 and t = 0.3; 0.4 and 0.5 for the different depths t. Thus, for example, results for the grating reflector 10 of Fig. 3b ) an aspect ratio of 1. In the example shown of Fig. 3 The grating reflectors 10 have an elliptical geometry with a diameter a = 0.4 of the aperture 24 and the vertex s = 0.45, wherein the sub-wavelength grating 20 is arranged centrally on the bottom 16. In Fig. 3 the marginal rays of the incident and the diffracted radiation are drawn, which are first diffracted at the sub-wavelength grating 20 in the zeroth order and then deflected by the side wall 18 of the microreflector 22.

The transmitted radiation is in Fig. 3 not shown because the semi-transparent sub-wavelength grating 20 transmits the drawn, incident light without deflection. When using an opaque sub-wavelength grating no transmission of incident light takes place, so that the comments on the FIGS. 3 and 4 also apply by way of example to such embodiments.

In this example, the rays each relate to a fixed point of incidence on the subwavelength grating 20. This shows that the acceptance angle of the incident light and the light scattering depend on the depth t, as shown by way of example in the table below. Depth t Acceptance angle of incident light Divergence of the reflected light 0.3 12.5 ° 20.5 ° 0.4 11.0 ° 16.3 ° 0.5 9.5 ° 15.1 °

The numbers indicate that as the depth t of the microreflector 22 increases, the divergence of the deflected radiation decreases; H. is focused to the viewer, since the exit angle describes the bundling.

It can be seen that the radiation reflected at the subwavelength grating 20 and at the microreflector 22 undergoes significant scattering. Furthermore, the acceptance angle of the incident light and the divergence of the reflected light decrease for increasing depths t of the micro-reflector 22. Finally, it should be mentioned that all the impact points on the sub-wavelength grating 20 must be considered to characterize the overall scattering of a grating reflector 10.

In the Fig. 3 Variation of the reflection properties of the microreflectors 22, explained by way of example with reference to the depth t, is one of several possibilities for changing the optical properties of the grating reflectors 10 and thus modulating the pattern of grating reflectors 10 and ultimately producing image information. Variation can also be achieved by changing other geometry parameters of the grating reflectors 10 from sub-wavelength grating 20 and micro-reflector 22. Now for the same reflector geometry as in Fig. 3b the diameter a of the aperture 24 varies. In Fig. 4 are three microreflectors 22 with a sub-wavelength grating 20 for the diameters a = 0.3; 0.4 and 0.5 of the apertures 24 in which the depth t = 0.4 is constant. In Fig. 4 The edge rays of the incident and the diffracted radiation are also drawn, which are first diffracted at the sub-wavelength grating 20 in the zeroth order and then deflected by the micro-reflector 22. The impact point selected in this example is shifted to the left with respect to the symmetry axis of the micro-reflector 22. The acceptance angles of the incident light and the light scattering of the reflected light are summarized in the following table: Diameter a Acceptance angle of incident light Divergence of the reflected light 0.3 17.0 ° 30.1 ° 0.4 15.5 ° 25.1 ° 0.5 14.0 ° 21.5 °

The variation of the diameter a of the aperture 24 also leads to a variation of the acceptance angle of the incident light and to a change in the divergence of the reflected light.

The pattern of many juxtaposed grid reflectors 10 can be used to represent subjects in a security element 12. It is z. B. two variants of structural geometries, preferably a different reflector geometry, in the respective areas before. Fig. 5 shows such a varied arrangement of grating reflectors 10, in which the depth t is selected differently in the areas I and II. The diameter of the aperture 24 of the microreflectors 22 is designated by a, the extent of the subwavelength gratings 20 is denoted by b. The depth t of the microreflectors 22 in the regions I and II is t ₁ and t _2, respectively. The pattern of the grating reflectors 10 allows a variety of different reflector geometries. Exemplary are the micro-reflectors 22in Fig. 5a with a parabolic and in Fig. 5b represented with a conical shape.

The microreflectors 22 may have a non-rotationally symmetric and / or asymmetric shape in cross section. Examples of unilaterally parabolic microreflectors 22 and microreflectors 22 having a trapezoidal cross section without curvature are shown in FIG Fig. 6 shown.

The shape of the aperture 24 can also be designed differently. Fig. 7 shows some examples of grating reflectors 10 having the following aperture shapes: square, circular and hexagonal. The grid reflectors 10 of Fig. 7a and Fig. 7b are arranged orthogonally. An optimal area coverage is achieved by a hexagonal arrangement, as in Fig. 7c is shown.

The location of the subwavelength gratings 20 within the individual microreflectors 22 may be shifted laterally. A continuous shift of the subwavelength gratings 20 relative to the aperture 24 of the microreflectors 22 can be used in particular for generating moiré effects, stereoscopic effects or running effects.

The generation of stereograms by means of the grating reflectors 10 is based on the in Fig. 8 illustrated example explained. Stereograms require at least two images, one for left-eye perception and the other for right-eye perception. Such stereograms can be generated by the grating reflectors 10 described herein both in reflection and in transmission.

Fig. 8 shows a security element 12 in the form of a stereogram. The area of the two letters "AB" is filled with a plurality of grid reflectors 10. In contrast, the surrounding background contains a homogeneous sub-wavelength grating 20. The incident light is directed in the exemplary embodiment by a semicylindrical reflector geometry in different directions. The grid reflectors 10 in the letter "A" are oriented so that they preferentially direct the light to the right eye. In contrast, the light reflected by the grille reflectors in the letter "B" is primarily perceived by the left eye. The orientation is determined by the side walls 18 of the micro-deflectors 22. With a correspondingly small pixelization, these two symbols can also be nested one inside the other.

Due to the large number of available geometry parameters that can be varied laterally, the grating reflectors described here provide 10 a wide range of possible designs and symbols. The desired color is accomplished by the appropriate selection of the grating period of the sub-wavelength grating 20.

By the pattern with many grating reflectors 10 also colored symbols or images can be generated, which are visible at a certain angle. In order to produce a color or an intensity contrast in the motif, for this purpose at least one of the above-mentioned parameters of the grating reflectors 10 in the pattern is laterally varied. Since the grating reflectors 10 have strongly angle-dependent properties, in particular parallax images can also be generated thereby. As a result, both parallactic movements and spatial effects can be realized.

Fig. 9 schematically shows a security element 12 with motifs, which are formed by a pattern of grating reflectors 10 in front of a homogeneous background. This surrounding area is filled in the exemplary embodiment with a homogeneous sub-wavelength grating 20. Due to the light deflection through the grating reflectors, the motif "star" is visible in oblique view in transmission, while the background appears dark ( Fig. 9a ).

Arrangements are also conceivable which show different motives in transmission at different viewing angles. Fig. 9b shows a motif with the letters "A" and "B", which are formed by differently shaped asymmetric grating reflectors 10. In the embodiment, the curved side wall 18 (second part 28; Fig. 2 ) at the letter "A" below, at the letter "B", the second part 28 of the side wall 18 in the micro-reflector 22 is arranged above. As already explained above, here a part of the transmission in the range of "A" becomes deflected above, while a portion of the transmitted light in the range of "B" is directed downward. The observer therefore perceives these motifs at different viewing angles in transmission. Also in this embodiment, the motifs with differently oriented grating reflectors 10 can be nested. The different motifs can then be perceived one after the other and depending on the subject at the same location of the security element when tilting a pattern designed in this way.

This in Fig. 9 shown security element 12 is used in particular in see-through windows of banknotes use. However, it is also possible to use only the reflection properties of this structure, for example for a security thread or a security strip (eg LEAD (Longlasting Economical Anticopy Device) strip).

As the subwavelength grating 20, not only one-dimensional periodic gratings can be used. Also, the subwavelength gratings need not be combined with microreflectors 22 that are curved in the same spatial direction. There are also sub-wavelength grating 20 conceivable, which are rotated about an axis of symmetry of the micro-reflector 22, wherein the rotation is varied within the pattern.

In the case of two-dimensional periodic subwavelength gratings 20, cross gratings are particularly suitable, the grating period preferably being perpendicular to the curvatures of a microreflector having a rectangular aperture 24. Circular gratings, however, are particularly suitable for reflectors with round apertures 24. Of course, also hybrid forms or elliptical subwavelength gratings 20 in microreflectors 22 with elliptical apertures 24 are possible.

In order to generate the motifs, grating reflectors 10 formed in each case from subwavelength grating 20 and microreflector 22 are arranged next to one another, their configuration varying laterally in order to form an image, for example, as a colored symbol. The variation of the diffraction characteristics of the subwavelength gratings 20, or the position of the subwavelength gratings 20 to the respective associated microreflector 22 or the reflection characteristic of the microreflectors 22, the alignment of the microreflectors 22, or the grating period of the subwavelength grating 20, or a combination thereof, causes a color or intensity contrast in the Motive.

Between adjacent grid reflectors 10 webs may remain in the substrate 14, resulting in land areas, which lie between the individual grid reflectors 10. This is not shown in the figures. The web surfaces are preferably formed by flat surfaces. In an exemplary construction, the web surfaces are laterally structured such that some web surfaces are provided with a metallization, but others are not.

By only partially metallization of the web surfaces two areas are created in which the web surfaces are provided with metal layer or without metal layer. The metallization causes incident radiation, which strikes a metallized land surface, to be thrown back as a reflex. In areas where the land areas are not metallized, however, the incident radiation is not reflected. The security element 12 is structured in this way in two areas that differ in their reflection behavior and also in their transmission behavior. As a result of the lateral structuring of the metallization, additional information can thus be coded in the security element 12. Instead of a metallization It is also possible to use a different reflection layer.

Instead of a metallization or reflection layer, an absorption layer can also be formed. The lateral structuring of the web surfaces then has no effect on the reflection behavior, but on the transmission behavior.

The security element 12 of the Fig. 1 used principle provides a large building block with which patterns or symbols can be designed by grating reflectors 10. The variation of the grating reflectors 10 in the pattern can be realized by many parameters.

The structure of the security element 12 can be made very easily in a single embossing process. All you need is a corresponding embossing tool that has a corresponding negative shape for each grid reflector 10, which thus generates both the microreflectors 22 and the sub-wavelength gratings 20. In order to achieve a high yield of the grating reflectors 10, the subwavelength gratings 20 must be precisely formed in the embossing tool. Specifically, the fabrication of the original of the sub-wavelength grating 20 can be accomplished by electron beam writing or interferometric techniques.

The microreflectors 22 typically have a depth t of 2 to 30 microns, a particularly preferred range is between 5 and 20 microns. Lower depths t are advantageous both with regard to the production of the embossing tool and the subsequent duplication. However, the sub-wavelength grating 20 is optically particularly effective when at least 4 to 10 grating periods can be accommodated at the bottom 16. This gives one lower limit for the size of the microreflectors 22 before. An upper limit arises when one wants to use the grating reflectors 10 as pixels, which should naturally be as small as possible and in particular should no longer be resolved with an unarmed eye. On the other hand, as flat an embossing as possible is advantageous in copying.

In the production of the original, the microreflectors 22 are first produced in a variant, for example photolithographically by direct laser writing. Independently, the structure of the subwavelength gratings 20 is generated. These two processes are preferably carried out accurately in one and the same photoresist, which is the basis for the embossing tool as a template. Alternatively, two different Belackungsvorgänge are possible.

It is also possible in a variant to first produce a homogeneous sub-wavelength grating 20, to lacquer it and then laterally to produce differently shaped micro-reflectors 22, for example by laser writing. For this purpose, first a sub-wavelength grating 20, for example by means of an electron beam exposure method, produced and molded. Subsequently, this grid substrate with a photoresist in sufficient thickness, for. B. 10 microns coated. Then microreflectors 22 are imprinted in the direct exposure process with a laserwriter, so that the subwavelength lattice 20 is exposed in the middle of the individual microreflectors after development.

The laserwriters used can work with 2-photon absorption processes. It is then possible to generate microreflectors 22, such as sub-wavelength gratings 20, in a single process.

The template produced for the embossing tool is now copied electroplated or in a nanoimprint process.

Since a plurality of security elements 12 are usually to be produced in one embossing step in the embossing process of a film, it is preferable to provide a plurality of adjacent stamp elements in the embossing tool, each of which has been produced from a template in the above-described manner.

The embossed film is preferably coated with an opaque metal layer. For this purpose, sputtering, electron beam evaporation or thermal evaporation come into question. Particularly suitable metals are aluminum, silver, gold, nickel or chromium or alloys of these materials. But there are also multi-layer structures in question, which contain at least one layer of metallic material. The thicknesses of the metal layer are between 20 and 100 nm. The metal surface is then finally preferably coated with a protective layer or laminated with a cover film.

To produce a laterally structured coating of the web surfaces, a corresponding coating is preferably first applied to all web surfaces and then removed again from some web surfaces. In the case of metallization, the removal is performed by demetallization. In this case, an etching process or the metal transfer process according to WO 2011/138039 A1 come into use. It is also possible to remove the layer, for example demetallization with the aid of ultrashort pulse lasers and the use of a writing laser beam.

When removing the layer, it is possible to selectively prevent a metallization in the area of the grating reflectors 10 from being removed. Thus, it is possible to completely fill the metallized structure with a photoresist, ie level. Then you can etch the photoresist to the levels of the land surfaces 11 and structure the thus exposed coating, for example, etch a metal layer. Subsequent removal of the photoresist then exposes the grating reflectors 10 again, which in this manner are not affected by engagement with the land surface 11.

LIST OF REFERENCE NUMBERS

10

grating reflector

12

security element

14

substratum

16

ground

18

Side wall

20

subwavelength

22

microreflector

24

aperture

26

first part

28

second part

a

diameter

b

expansion

t

depth

Claims (21)

Security element for an object to be protected, such. A security paper, document of value or the like comprising a substrate (14) having a plurality of microreflectors (22) arranged in a pattern and microstructures, wherein - each of the microstructures (20) is assigned to one of the microreflectors (22), whereby a plurality of grating reflectors (10) comprising in each case a microreflector (22) and at least one of the microstructures (20) are formed, each of the microstructures (20) is adapted to reflectively diffract visible radiation incident through an aperture (24) of the microreflector (22) from a hemisphere, and the grid reflectors (10) arranged in the pattern produce an image perceptible by a viewer, characterized in that - Each microstructure is formed as a sub-wavelength grating (20), which reflects a first part of the visible radiation, which is incident through the aperture (24) of the microreflector (22) from the half-space, reflective, wherein each of the subwavelength gratings (20) diffracts the first part into a zeroth reflection diffraction order. A security element according to claim 1, characterized in that the sub-wavelength grating (20) is formed as a semitransparent sub-wavelength grating which transmissively diffracts a second portion of the visible radiation incident through the aperture (24) of the micro-reflector (22) from the hemisphere, each one the sub-wavelength grating (20) diffracts the second part into a zeroth transmission diffraction order. A security element according to claim 2, characterized in that in each grating reflector (10) the subwavelength grating (20) and the microreflector (22) are matched to one another such that the subwavelength grating (20) passes through the aperture (24) of the microreflector (22) out of the Half-space incident and reflected by the microreflector (22) radiation in the zeroth transmission diffraction order. A security element according to claim 1, characterized in that the sub-wavelength grating (20) is formed as an opaque sub-wavelength grating. A security element according to any one of claims 1 to 4, characterized in that in each grating reflector (10) the sub-wavelength grating (20) and the micro-reflector (22) are matched to one another that the micro-reflector (22) from the sub-wavelength grating (20) in the zeroth Reflection diffraction order diffracted radiation at least partially reflected as re-radiation in the half space back. Security element according to one of claims 1 to 5, characterized in that the microreflectors (22) are each formed as a concave mirror or grooves with a flat bottom (16), wherein the sub-wavelength grating (20) is arranged on the flat bottom (16), preferably the microreflectors (22) adjacent to the bottom (16) have a side wall (18) which is parabolic in cross-section, straight, elliptical or formed as a hybrid form thereof. A security element according to any one of the preceding claims, characterized in that the micro-deflectors (22) are asymmetrical in cross-section and have a bottom (16) and at least one side wall (18) in particular a first part (26) of the side wall (18) is straight and a second part (28) of the side wall (18) is concave, in particular parabolic or elliptical, in particular that a first part (26) of the side wall (18) perpendicular to the floor (16) is. Security element according to one of the above claims, characterized in that each sub-wavelength grating (20) has a grating period between 100 nm and 500 nm, in particular between 240 nm and 420 nm. Security element according to one of the above claims, characterized in that the microreflectors (22) and the subwavelength gratings (20) are coated on their side facing the half-space with at least one metallic layer, preferably with Al, Ag, Au, Cu, Cr or one of these Alloy containing metals, and / or that the micro-reflectors (22) and the sub-wavelength gratings (20) on its side facing the half-space a trilayer coating of two superimposed metal or semiconductor layers having an intervening dielectric layer. A security element according to any one of the preceding claims, characterized in that the aperture (24) has a diameter (a) and the microreflector (22) has a depth (t) from the aperture (24) to a bottom (16) within of the pattern at least one of the following properties of the grating reflectors (10) varies to produce the image: location of the subwavelength gratings (20) to the respective associated microreflector (22), diameter (a) of the aperture (24), depth (t) of the Microreflector (22), alignment of the microreflector (22), shape of the microreflector (22) and / or grating period of the subwavelength grating (20), where preferred the property of the grating reflectors (10) within the pattern is continuously varied so that the image is produced by moiré effect. Security paper or document of value, characterized by a security element (12) according to one of claims 1 to 10. Method for producing a security element (12) for an object to be protected, such as e.g. As a security paper, document of value or the like, wherein on a substrate (14) a plurality of micro-deflectors arranged in a pattern (22) and a plurality of microstructures are formed, wherein - each of the microstructures (20) is assigned to one of the microreflectors (22), whereby a plurality of grating reflectors (10) comprising in each case a microreflector (22) and at least one of the microstructures (20) are formed, each of the microstructures (20) is formed to reflectively diffract visible radiation incident through an aperture (24) of the microreflector (22) from a hemisphere, and the grid reflectors (10) arranged in the pattern produce an image perceptible by a viewer, characterized in that - Each microstructure is formed as a sub-wavelength grating (20) which reflects a first part of the visible radiation, which is incident through the aperture (24) of the microreflector (22) from the half-space, reflective, wherein each of the subwavelength gratings (20) diffracts the first part into a zeroth reflection diffraction order. A method according to claim 12, characterized in that the sub-wavelength grating (20) formed as a semi-transparent sub-wavelength grating which diffractively diffracts a second portion of the visible radiation incident through the aperture (24) of the microreflector (22) from the half space, each of the subwavelength gratings (20) diffracting the second portion into a zeroth transmission diffraction order. A method according to claim 12 or 13, characterized in that in each grating reflector (10) the sub-wavelength grating (20) and the micro-reflector (22) are matched to one another that the sub-wavelength grating (20) through the aperture (24) of the microreflector (22). radiation incident from the half-space and reflected by the microreflector (22) is transmitted to the zeroth transmission diffraction order. A method according to claim 12, characterized in that the sub-wavelength grating (20) is formed as an opaque sub-wavelength grating. Method according to one of claims 12 to 15, characterized in that in each grating reflector (10), the sub-wavelength grating (20) and the micro-reflector (22) are matched to one another that the micro-reflector (22) from the sub-wavelength grating (20) in the zeroth Reflection diffraction order diffracted radiation at least partially reflected as re-radiation in the half space back. Method according to one of claims 12 to 16, characterized in that the micro-reflectors (22) are each formed as concave mirrors or grooves with a flat bottom (16), wherein the sub-wavelength grating (20) is arranged on the flat bottom (16), wherein preferably the microreflectors (22) next to the bottom (16) are configured having a side wall (18) which is formed in cross-section parabolic, straight, elliptical or as a hybrid form thereof. Method according to one of claims 12 to 17, characterized in that the micro-deflectors (22) are asymmetrical in cross-section and with a bottom (16) and at least one side wall (18) are configured, in particular a first part (26) of the side wall ( 18) straight and a second part (28) of the side wall (18) are concave, in particular parabolic or elliptical, in particular, that a first part (26) of the side wall (18) perpendicular to the bottom (16) is configured. Method according to one of claims 12 to 18, characterized in that each sub-wavelength grating (20) is formed with a grating period between 100 nm and 500 nm, in particular between 240 nm and 420 nm. Method according to one of claims 12 to 19, characterized in that the microreflectors (22) and sub-wavelength gratings (20) are coated on their side facing the half-space with at least one metallic layer, preferably with Al, Ag, Au, Cu, Cr or a alloy containing these metals. Method according to one of Claims 12 to 20, characterized in that the aperture (24) has a diameter (a) and the microreflector (22) has a depth (t) from the aperture (24) to the bottom (16) within a pattern, at least one of the following properties of the grating reflectors (10) is varied to produce the image: location of the subwavelength gratings (20) to the respective associated microreflector (22), diameter (a) of the aperture (24), depth (t ) of the microreflector (22), arrangement of the microreflector (22), shape of the microreflector (22) and / or grating period of the subwavelength grating (20), preferably the property the grating reflectors (10) within the pattern is continuously varied so that the image is generated by moiré effect.

Images