DE102015016751A1 - Security element for security papers, documents of value 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. For example, a security paper, document of value or the like, which has a plurality of micro-reflectors in a pattern and a plurality of microstructures, which generate together with the micro-reflectors an image perceivable 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, identity 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 in detail in the prior art, for example in US Pat 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 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 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 into 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 grating is designed such that it diffractively diffracts visible radiation, ie light incident through an aperture of the microreflector from the half space, into a zeroth reflection diffraction order, with the subwavelength grating and the microreflector being preferably matched to one another for each grating reflector the microreflector reflects back at least a portion of the radiation diffracted by the sub-wavelength grating into the zeroth reflection diffraction order as re-radiation into the half-space.

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, 50%, 60% or 70% of the incident radiation is in the zeroth diffraction order, 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 in that the security element displays an image or several images to an observer, both when it is in the same half-space as the illumination for the security element (reflected image or supervisory image). and also when the security element and the observer lighting are in two different half-spaces with respect to the security element (transmitted image or transparent image). 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 for certain wavelengths which lead to light absorption, as for example in the documents DE 10 2011 101 635 A1 and DE 10 2009 056 933 A1 described, the disclosure content 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 is an 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. H. 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. Preferably, a plurality of grating reflectors with a different design (combination of microreflector and microstructure in the respective grating reflector) are provided so that there is a difference in color or intensity between the grating reflectors, which overall produces 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 sub-wavelength grating and associated microreflector is specified by the corresponding 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 that actually cut the sphere or 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 aligned in a first direction so that these grating reflectors are visible in a first viewing direction. At 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, ie. H. 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. It is further preferred that each subwavelength grating has a grating period between 100 nm and 500 nm, in particular 240 nm to 420 nm, wherein the subwavelength grating can be periodically formed one-dimensionally periodically or even two-dimensionally. 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 semitransparent sub-wavelength grating which comprises a second part of the visible radiation passing through the aperture ( 24 ) of the microreflector ( 22 ) is incident from the half-space, transmittently diffracting 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 preferred 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 for original production, such as electron beam or focussed ion beam exposure methods, allow an even finer configuration of the geometry and are therefore particularly suitable for producing the 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.

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

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

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

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

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

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

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;

8th a security element of an array of microreflectors with subwavelength gratings, in which the microreflectors are oriented differently depending on the group; and

9 the emergence of the image elicited by the security element.

The 1 to 9 show grating reflectors 10 a security element 12 , The security element 12 is from a substrate 14 , For example, a transparent film, made at the top (the term is purely exemplary and should not specify a preferred direction) is formed an embossed structure. The embossed structure comprises a multiplicity of microreflectors 22 used in the embodiments of 1 to 4 as elliptical reflectors with a flat bottom 16 are formed. In other embodiments, for example, in which the 5 and 6 the micro-reflectors are designed in other geometries, in particular not rotationally symmetrical.

Every grid reflector 10 thus indicates the ground 16 and at least one sidewall 18 on. A semi-transparent sub-wavelength grating 20 is on the ground 16 of the grating reflector 10 intended. The at least one side wall 18 and the ground 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 ground 16 to the aperture 24 determines the depth t of the grating reflector 10 , The substrate 14 is in the area of the soil 16 at least partially transparent.

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

The proportion of radiation from the half-space incident on the sub-wavelength grating 20 falls, is reflected directly there and transmitted to a second part. The other remainder of the visible radiation incident to it from the microreflector 22 to the subwavelength grid 20 is reflected from the sub-wavelength grating 20 diffracted in zeroth diffraction order also reflected to a part and transmitted to a part. 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 amount of light that is first on the microreflector 22 is thus reflected differently on 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 that is deflected laterally can be reduced by providing an asymmetrical grating reflector 10 be increased, as in 2a and 2 B is shown. A first part 26 the side wall 18 is straight in cross-section and is in particular perpendicular to the ground 16 , A second part 28 the side wall 18 which the first part 26 is arranged opposite, is as in 1 parabolic shaped. The ratio of diameter a of the aperture 24 to depth t of the grating reflector 10 is 1 in 2a and 0.53 in 2 B ,

Like this in 2a and 2 B represented by the left ray of light, the first part shadows 26 the side wall 18 a part of the subwavelength grating 20 in the incident radiation. This increases the proportion of light coming from the grating reflector 10 is distracted. The smaller the depth t of the grating reflector 10 is, the lower the shaded area, so that the deflected radiation component decreases with decreasing depth t.

3 shows three different depth grille reflectors 10 with constant diameter a of the aperture 24 and in each case the same sub-wavelength grating 20 on the ground 16 of the grating reflector 10 , The following dimensionless values can be assigned to this figure: 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 the 3b) an aspect ratio of 1. In the example shown of 3 own the grid reflectors 10 an elliptical geometry with a diameter a = 0.4 of the aperture 24 and the vertex s = 0.45, where the sub-wavelength grating 20 centric on the ground 16 is arranged. In 3 the marginal rays of the incident and the diffracted radiation are drawn, which first at the sub-wavelength grating 20 bent in the zeroth order and then through the sidewall 18 of the microreflector 22 to get distracted.

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

In each case, the beams relate to a fixed impact point on the sub-wavelength grating 20 , Here it can be seen 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 show that with increasing depth t of the microreflector 22 the divergence of the deflected radiation decreases, ie it is concentrated to the viewer, because the exit angle describes the bundling.

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

In the 3 Variation of the reflection properties of the microreflectors 22 , explained by way of example on the basis of the depth t, is one of several possibilities, the optical properties of the grating reflectors 10 to change and thus the pattern of grid reflectors 10 to modulate and ultimately to generate image information. A variation can also be achieved by changing other geometry parameters of the grating reflectors 10 from sub-wavelength grating 20 and microreflector 22 be achieved. Now for the same reflector geometry as in 3b the diameter a of the aperture 24 varied. In 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 shown where the depth t = 0.4 is constant. In 4 are also the marginal rays of the incident and the diffracted radiation located, which first at the sub-wavelength grating 20 bent in the zeroth order and then through the microreflector 22 to get distracted. The impact point selected in this example is to the left with respect to the symmetry axis of the micro-reflector 22 postponed. 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 be used. It is z. B. two variants of structural geometries, preferably a different reflector geometry, in the respective areas before. 5 shows such a varied arrangement of grating reflectors 10 in which the depth t is chosen differently in the areas I and II. The diameter of the aperture 24 the microreflectors 22 is a, the extent of the subwavelength gratings 20 is denoted by b. The depth t of the microreflectors 22 is in the ranges I and II t ₁ and t ₂ . The pattern of the grid reflectors 10 allows a variety of different reflector geometries. Exemplary are the micro-reflectors 22 in 5a with a parabolic and in 5b represented with a conical shape.

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

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

The location of the subwavelength gratings 20 within each microreflector 22 can be shifted laterally. A continuous shift of the subwavelength gratings 20 relative to the aperture 24 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 determined by the in 8th 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 achieved by the grating reflectors described herein 10 be generated both in reflection and in transmission.

8th shows a security element 12 in the form of a stereogram. The area of the two letters "AB" is with a variety of grid reflectors 10 filled. By contrast, the surrounding background contains a homogeneous sub-wavelength grating 20 , The incident light is directed in the embodiment by a semi-cylindrical 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 through the side walls 18 the microreflectors 22 certainly. 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 broad design possibility of patterns and symbols. The desired color is determined by the appropriate selection of the grating period of the sub-wavelength grating 20 accomplished.

Through the pattern with many grid reflectors 10 You can also create colored symbols or images that are visible at a certain angle. In order to produce a color or an intensity contrast in the subject, this is at least one of the above-mentioned parameters of the grating reflectors 10 varies laterally in the pattern. Because the grid reflectors 10 have strong angle-dependent properties, in particular also parallax images can be generated. As a result, both parallactic movements and spatial effects can be realized.

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

Arrangements are also conceivable which show different motives in transmission at different viewing angles. 9b shows a motif with the letters "A" and "B", which are of differently designed asymmetric grating reflectors 10 are formed. In the embodiment, the curved side wall is located 18 (second part 28 ; 2 ) at the letter "A" below, at the letter "B" is the second part 28 the side wall 18 in the microreflector 22 arranged above. As already explained above, part of the transmission in the region of "A" is deflected upwards while a part of the transmitted light in the region of "B" is directed downwards. The observer therefore perceives these motifs at different viewing angles in transmission. Also in this embodiment, the motifs with differently oriented grating reflectors 10 be nested inside each other. 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 9 illustrated security element 12 finds particular use 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 a subwavelength grating 20 not only one-dimensional periodic gratings can be used. The subwavelength gratings do not have to be with microreflectors 22 be combined, which are curved in the same spatial direction. They are also subwavelength gratings 20 conceivable, about an axis of symmetry of the micro-reflector 22 are twisted, wherein the rotation is varied within the pattern.

For two-dimensional periodic subwavelength gratings 20 Cross gratings are particularly suitable, the grating period preferably being perpendicular to the curvatures of a microreflector with a rectangular aperture 24 runs. Circular gratings, on the other hand, are particularly suitable for reflectors with round apertures 24 suitable. Of course, also hybrid forms or elliptical subwavelength gratings are 20 in micro-reflectors 22 with elliptical apertures 24 possible.

To generate the motifs are each made of sub-wavelength grating 20 and microreflector 22 formed grating reflectors 10 arranged side by side, wherein their configuration varies laterally to form an image, for example, as a colored symbol. The variation of the diffraction properties of the subwavelength gratings 20 , or the location of the subwavelength gratings 20 to the respective associated microreflector 22 or the reflection property of the microreflectors 22 , the orientation of the micro-reflectors 22 , or the grating period of the subwavelength grating 20 or a combination of these causes a color or intensity contrast in the subject.

Between adjacent grid reflectors 10 can webs in the substrate 14 remain, resulting in web areas, which between the individual grid reflectors 10 lie. 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 into two areas that differ in their reflection behavior and also in their transmission behavior. Due to the lateral structuring of the metallization can thus in the security element 12 additional information is encoded. Instead of a metallization, a different type of reflection layer can also be used.

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.

That in the security element 12 of the 1 used principle provides a large building block, with the pattern or symbols through grille reflectors 10 can be designed. 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 easily produced in a single stamping process. For this you only need a corresponding embossing tool that has a corresponding negative shape for each grid reflector 10 that has both the microreflectors 22 as well as the subwavelength gratings 20 generated. To a high yield of the grating reflectors 10 To achieve this, the subwavelength gratings must be used 20 be precisely formed in the stamping tool. Especially the production of the original of the subwavelength grating 20 can be achieved by electron beam writing or interferometric methods.

The micro-reflectors 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 unfolds 20 Its effect is visually particularly good, if at least 4 to 10 grid periods on the ground 16 can be accommodated. This gives a lower limit to the size of the microreflectors 22 in front. An upper limit arises when you use the grid reflectors 10 would like to use as pixels, which should naturally be as small as possible and in particular should not be resolved with unarmed eye. On the other hand, as flat an embossing as possible is advantageous in copying.

In the production of the original, in a variant, first the micro-reflectors 22 produced, for example, photolithographically by direct laser writing. Regardless, the structure of the subwavelength gratings becomes 20 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, first a homogeneous sub-wavelength grating 20 to produce, to lacquer this and then laterally differently shaped micro-reflectors 22 to generate, 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 become micro-reflectors 22 imaged in a direct exposure process with a laserwriter, so that the sub-wavelength grating 20 in the middle of each microreflector after development is exposed.

The laserwriters used can work with 2-photon absorption processes. It is then possible micro-reflectors 22 like subwavelength gratings 20 in a single process.

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

As in the embossing process of a film usually a variety of security elements 12 are to be produced in a stamping step, it is preferable to provide in the embossing tool a plurality of juxtaposed stamp elements, which were each produced in the manner described above from a template.

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 one can specifically prevent a metallization in the area of the lattice reflectors 10 Will get removed. Thus, it is possible to completely fill the metallized structure with a photoresist, ie level. Then you can use the photoresist down to the levels of the web surfaces 11 Etch and structure the thus exposed coating, for example, etch a metal layer. Subsequent removal of the photoresist then deposits the grating reflectors 10 free again in this way from engaging the bridge surface 11 are not affected.

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

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]
• EP 1182054 B1 [0005]
• WO 2011/029602 A2 [0005]
• WO 2002/101669 A2 [0005]
• EP 1476317 A1 [0005]
• EP 1893074 A2 [0005]
• WO 2010/136339 A2 [0005]
• WO 2011/012460 A2 [0005]
• WO 2014/012667 A1 [0008, 0018]
• DE 102011101635 A1 [0019]
• DE 102009056933 A1 [0019]
• WO 2011/138039 A1 [0045, 0108]

Claims (24)

Security element for an object to be protected, such. A security paper, document of value or the like, which is a substrate ( 14 ) with a plurality of microreflectors arranged in a pattern ( 22 ) and microstructures, wherein - each of the microstructures ( 20 ) one of the micro-reflectors ( 22 ), whereby several grating reflectors ( 10 ) comprising in each case a microreflector ( 22 ) and at least one of the microstructures ( 20 ), - each of the microstructures ( 20 ) is designed to transmit visible radiation through an aperture ( 24 ) of the microreflector ( 22 ) is incident from a half-space, bends in a reflective manner, and - the grating reflectors arranged in the pattern ( 10 ) generate an image perceptible by a viewer, characterized in that - each microstructure is designed as a subwavelength grating ( 20 ), which forms a first part of the visible radiation passing through the aperture ( 24 ) of the microreflector ( 22 ) is incident from the half-space, diffractively diffracting, - each of the sub-wavelength grids ( 20 ) diffracts the first part into a zeroth reflection diffraction order. Security element according to claim 1, characterized in that the sub-wavelength grating ( 20 ) is formed as a semitransparent sub-wavelength grating which comprises a second portion of the visible radiation passing through the aperture ( 24 ) of the microreflector ( 22 ) is incident from the half space, transmissively diffracts, each of the subwavelength gratings ( 20 ) diffracts the second part into a zeroth transmission diffraction order. Safety element according to claim 2, characterized in that in each grid reflector ( 10 ) the sub-wavelength grating ( 20 ) and the microreflector ( 22 ) are coordinated so that the sub-wavelength grating ( 20 ) through the aperture ( 24 ) of the microreflector ( 22 ) incident from the half-space and from the microreflector ( 22 ) transmits reflected radiation to the zeroth transmission diffraction order. Security element according to claim 1, characterized in that the sub-wavelength grating ( 20 ) is formed as an opaque sub-wavelength grating. Security element according to one of claims 1 to 4, characterized in that in each grid reflector ( 10 ) the sub-wavelength grating ( 20 ) and the microreflector ( 22 ) are coordinated so that the microreflector ( 22 ) from the subwavelength grid ( 20 ) reflected radiation in the zeroth reflection diffraction order at least partially reflected backward in the half space back. Security element according to one of claims 1 to 5, characterized in that the microreflectors ( 22 ) each as a concave mirror or gutters with a flat bottom ( 16 ), wherein the sub-wavelength grating ( 20 ) on level ground ( 16 ), wherein preferably the microreflectors ( 22 ) next to the ground ( 16 ) a side wall ( 18 ), which is parabolic in cross-section, straight, elliptical or formed as a hybrid form thereof. Security element according to one of the above claims, characterized in that the microreflectors ( 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 ) straight and a second part ( 28 ) of the side wall ( 18 ) is concave, in particular parabolic or elliptical. Security element according to claim 7, characterized in that a first part ( 26 ) of the side wall ( 18 ) perpendicular to the ground ( 16 ). Security element according to one of the preceding 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 an alloy containing these metals. Security element according to one of the above claims, characterized in that the microreflectors ( 22 ) and the subwavelength gratings ( 20 ) on its half-space side facing a Trilayer coating of two superimposed metal or semiconductor layers having an intervening dielectric layer. Security element according to one of the preceding claims, characterized in that the aperture ( 24 ) has a diameter (a) and the microreflector ( 22 ) from the aperture ( 24 ) to a floor ( 16 ) has a depth (t), wherein within 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 respectively assigned 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 sub-wavelength grating ( 20 ), wherein preferably the property of the grating reflectors ( 10 ) is continuously varied within the pattern 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 12. Method for producing a security element ( 12 ) for an object to be protected, such. As a security paper, document of value or the like, wherein on a substrate ( 14 ) a plurality of microreflectors arranged in a pattern ( 22 ) and a plurality of microstructures are formed, wherein - each of the microstructures ( 20 ) one of the micro-reflectors ( 22 ), whereby several grating reflectors ( 10 ) comprising in each case a microreflector ( 22 ) and at least one of the microstructures ( 20 ), - each of the microstructures ( 20 ) is formed so that it transmits visible radiation passing through an aperture ( 24 ) of the microreflector ( 22 ) is incident from a half-space, bends in a reflective manner, and - the grating reflectors arranged in the pattern ( 10 ) generate an image perceptible by a viewer, characterized in that - each microstructure is designed as a subwavelength grating ( 20 ), which forms a first part of the visible radiation passing through the aperture ( 24 ) of the microreflector ( 22 ) is incident from the half-space, diffractively diffracting, - each of the sub-wavelength grids ( 20 ) diffracts the first part into a zeroth reflection diffraction order. Method according to claim 14, characterized in that the sub-wavelength grating ( 20 ) is formed as a semitransparent sub-wavelength grating which comprises a second portion of the visible radiation passing through the aperture ( 24 ) of the microreflector ( 22 ) is incident from the half space, transmissively diffracts, each of the subwavelength gratings ( 20 ) diffracts the second part into a zeroth transmission diffraction order. Method according to claim 14 or 15, characterized in that in each grid reflector ( 10 ) the sub-wavelength grating ( 20 ) and the microreflector ( 22 ) are coordinated so that the sub-wavelength grating ( 20 ) through the aperture ( 24 ) of the microreflector ( 22 ) incident from the half-space and from the microreflector ( 22 ) transmits reflected radiation to the zeroth transmission diffraction order. Method according to claim 14, characterized in that the sub-wavelength grating ( 20 ) is formed as an opaque sub-wavelength grating. Method according to one of claims 14 to 17, characterized in that in each grid reflector ( 10 ) the sub-wavelength grating ( 20 ) and the microreflector ( 22 ) are coordinated so that the microreflector ( 22 ) from the subwavelength grid ( 20 ) reflected radiation in the zeroth reflection diffraction order at least partially reflected backward in the half space back. Method according to one of claims 14 to 18, characterized in that the micro-reflectors ( 22 ) each as a concave mirror or gutters with a flat bottom ( 16 ), wherein the sub-wavelength grating ( 20 ) on level ground ( 16 ), wherein preferably the microreflectors ( 22 ) next to the ground ( 16 ) a side wall ( 18 ), which in cross section is parabolic, straight, elliptical or as a hybrid form thereof. Method according to one of claims 14 to 19, characterized in that the micro-reflectors ( 22 ) are asymmetrical in cross-section and with a bottom ( 16 ) and at least one side wall ( 18 ), in particular a first part ( 26 ) of the side wall ( 18 ) straight and a second part ( 28 ) of the side wall ( 18 ) are concave, parabolic or elliptical in particular. Method according to claim 20, characterized in that a first part ( 26 ) of the side wall ( 18 ) perpendicular to the ground ( 16 ) is configured. Method according to one of claims 14 to 21, 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 14 to 22, characterized in that the microreflectors ( 22 ) and 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 an alloy containing these metals. Method according to one of claims 14 to 23, characterized in that the aperture ( 24 ) has a diameter (a) and the microreflector ( 22 ) from the aperture ( 24 ) to the ground ( 16 ) has a depth (t), wherein 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 respectively assigned 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 sub-wavelength grating ( 20 ), wherein preferably the property of the grating reflectors ( 10 ) is varied so continuously within the pattern that the image is generated by moiré effect.

Images