CN107921810B

CN107921810B - Optically variable security element

Info

Publication number: CN107921810B
Application number: CN201680043728.XA
Authority: CN
Inventors: M.R.J.谢勒; J.席纳贝克
Original assignee: Quartech Currency Technology LLC
Current assignee: Quartech Currency Technology LLC
Priority date: 2015-05-27
Filing date: 2016-05-17
Publication date: 2020-01-14
Anticipated expiration: 2036-05-17
Also published as: EP3302995A1; DE102015006793A1; WO2016188619A1; CN107921810A; EP3302995B1

Abstract

The invention relates to an optically variable security element (12) for providing security to a valuable object, wherein a relief structure (22) and a two-layer colored mirror (30) are arranged one above the other and form an optically variable and colored appearance in reflection under the effect of the combined action, wherein the two-layer colored mirror (30) consists of a reflective metal layer (32) and an ultrathin absorption layer (34) arranged on the metal layer (32) and consisting of silicon, a silicon alloy or SiO with x < 1_xAnd (4) preparing.

Description

Optically variable security element

The invention relates to an optically variable security element for providing security to valuable articles. The invention also relates to a method for producing such a security element and to a data carrier having such a security element.

Data carriers, such as value documents or authentication documents or other valuable items, such as branded goods, are often provided for security purposes with security elements which enable the authenticity of the data carrier to be checked and which at the same time serve as protection against unauthorized copying. Security elements with viewing-angle-dependent effects play a special role in the authenticity assurance, since they cannot be reproduced even with the most advanced reproduction devices. The security element is designed here with optically variable elements which convey different image impressions to viewers at different viewing angles and, for example, display different color or brightness impressions and/or different pattern themes depending on the viewing angle.

Optically variable security elements are used in part with colored mirrors, i.e. with optical components that reflect light in the visible spectral range in a frequency-dependent and thus colored manner. For example, security elements are known which have a multi-layer lamellar element whose color impression differs for observers with different viewing angles. The color-tilting effect of such thin-layer elements is based on viewing-angle-dependent interference effects caused by multiple reflections in different sub-layers of the element. The path difference of the light reflected on the different layers depends on the one hand on the optical thickness of the dielectric spacer layer, which determines the spacing between the semitransparent absorbing layer and the reflecting layer, and on the other hand varies with the respective viewing angle. The dielectric spacer layer of such thin-layer elements typically has a relatively large thickness of between 200nm and 400nm and is therefore time-and cost-consuming to manufacture.

It is also known that a coloured mirror surface is constituted by a coating of a reflective metallic surface with a coloured translucent paint. However, such designs cannot be combined with a precisely fitting and colorless inscription element, or can only be combined with difficulty, since the paint layer cannot be machined or removed by the laser beam. Furthermore, the additional thickness of the reinforcing structure of the paint layer, which leads to a colored mirror surface, is often disadvantageous and is not desirable in particular in security elements for banknotes and other documents of value.

Starting from this, the object of the invention is to provide an optically variable security element which can be produced cost-effectively and has a high security against forgery and an attractive colored visual appearance, in particular in reflection.

The object is achieved according to the invention by the features of the independent claims. Improvements of the invention are the subject of the dependent claims.

According to the invention, provision is made for a security element of the type according to the invention,

the relief structure and the colored mirror surface of the double layer are arranged one above the other and form an optically variable and colored or colored appearance in reflection under the combined action, wherein,

the double-layer colored mirror consists of a reflective metal layer and an ultrathin absorption layer arranged on the metal layer, the ultrathin absorption layer consisting of silicon, a silicon alloy or SiO with x < 1_xAnd (4) preparing.

As will be explained in further detail later, the reflection color of the metal layer can be tuned by varying the thickness of the absorbing layer by means of a suitable ultra-thin absorbing layer. The silver reflection of the aluminum metal layer can be adjusted virtually steplessly from the silver reflection color of the pure metal layer via the gold, yellow and red reflection colors to the blue reflection color, for example by applying a silicon absorber layer with a layer thickness of between a few nanometers and about 35 nm. This color sequence is repeated at layer thicknesses of between about 40nm and 80nm, the reflection spectrum showing two or more reflection minima for larger layer thicknesses above 80nm, thus forming other color sequences.

Gold and copper-colored metal alloys can be produced particularly cost-effectively by means of the absorber layer which can be produced in a simple manner and has a small layer thickness. By means of the regions of the metal layer with different layer thicknesses or uncoated regions, different metal reflection colors can be combined with one another in a simple manner and with a high spatial resolution.

An ultra-thin absorption layer made of silicon has proved to be particularly advantageous, wherein crystalline silicon, polycrystalline silicon or amorphous silicon may be used. Furthermore, for example SiAl, SiFe, SiCu or SiTi and SiO_xThe silicon alloy of (1) is also suitable for ultra-thin absorber layers, where x<1. Preferably x<0.5, and x is particularly preferred<0.2。

The ultrathin absorption layer advantageously has a layer thickness of between 1nm and 200nm, preferably between 1nm and 100nm, particularly preferably between 5nm and 35 nm.

The metal layer of the double-layer coloured mirror is advantageously aluminum, silver, copper, tin, zinc, iron, chromium, nickel or an alloy of these metals. The layer thickness of the metal layer is advantageously between 10nm and 100nm, preferably between 15nm and 80 nm.

In an advantageous embodiment, the two-layer colored mirror comprises at least two subregions which have different layer thicknesses of the ultrathin absorption layer, including a layer thickness of zero. The colored mirror surface has a different reflection color in the at least two partial regions, so that the relief structure and the double-layer colored mirror surface form, under the effect of the interaction, an appearance which is at least two colors in reflection.

The partial regions of different layer thicknesses can be formed by these regions, in which the layer thickness of the ultra-thin absorber layer is in each case not equal to zero, but differs from one another. Such different thicknesses can be achieved, in particular, by masking when applying the absorption layer, by selective removal of the absorption layer by means of a photolithographic method or laser application, or by appropriate design of the relief structure in combination with appropriately selected conditions when applying the absorption layer.

Alternatively or additionally, the colored mirror can comprise openings which are present only in the ultrathin absorption layer and which constitute sub-regions of the colored mirror with an absorption layer having a layer thickness of zero. Within the context of the present description, such subregions are also to be regarded as openings in the absorption layer, in which subregions the optical properties of the absorption layer are changed such that the absorption layer loses its absorption properties, i.e. becomes transparent, for example by oxidation or other material changes.

The metal layer advantageously has a specular reflection in the openings that are present only in the ultra-thin absorbing layer. The metal layer has a reflectivity in the opening which is more than 80%, in particular more than 90%, of the reflectivity of an uncoated metal layer of the same type.

Alternatively or additionally, the colored mirror can comprise openings which extend through the absorption layer and the metal layer and thus constitute openings in the entire colored mirror. In the openings of the colored mirror, the base layer or the see-through areas in the security element can be seen.

In an advantageous development, the colored mirror is combined with a colored layer which is visible in the openings of the colored mirror. Preferably, the colored layer is present here on the side of the metal layer facing away from the absorption layer. By means of such a colored layer, the security element can combine one or more metallic reflection colors of the colored mirror with a non-metallic color impression in the opening.

The relief structure of the security element is advantageously formed by a diffractive structure, for example a hologram, a holographic grating image or a diffractive structure similar to a hologram. Achromatic structures, such as matt structures, micromirror devices, blazed gratings with sawtooth-like groove profiles or fresnel lens devices, or nanostructures, such as subwavelength structures, can also be considered as relief structures.

In an advantageous embodiment, the two-layer colored mirror is designed as a relief-structured coating. Here, both the metal layer and the absorption layer may be directed towards the relief structure. However, the color change of the absorption layer is only visible when viewed from the side of the absorption layer, so that the security element must be designed accordingly according to the desired viewing direction.

In an advantageous embodiment, the relief structure can be designed such that, when the absorption layer is applied, the relief structure leads to locally different layer thicknesses of the absorption layer and thus to different reflection colors. For example, the relief structure may comprise micromirrors with different inclinations, and the absorber layer may be applied on the micromirror plate by directional evaporation coating, in particular oblique evaporation coating. As explained in further detail below, the layer thickness of the applied absorber layer is closely related to the relative orientation of the micromirror surface and the evaporation coating direction, so that the desired color effect can be formed by a suitable orientation of the micromirror plate.

In other embodiments, the relief structure is a lenticular pattern comprising a plurality of lenticules, the lenticular pattern being spaced apart from the coloured mirrors. The colored mirror surface here advantageously contains one or more given or predetermined images which can be recognized from certain defined viewing directions when the colored mirror surface is viewed through the lenticular screen. The given image comprises, inter alia, apertures only present in the absorbing layer and/or apertures throughout the entire colored mirror surface. When viewed, the lenticular sheet and the coloured mirror cooperate to present one or more of said given images and thereby create an optically variable and coloured appearance in reflection.

What is referred to as a microlens in the context of the present description means a lens whose size in at least one lateral direction is below the resolution limit of the naked eye. The microlenses may be spherical or aspherical, for example, but cylindrical lenses are also conceivable. The diameter of the spherical or aspherical microlenses is preferably between 5 μm and 100 μm, in particular between 10 μm and 50 μm, particularly preferably between 15 μm and 20 μm. The width of the micro cylindrical lenses is preferably between 5 μm and 100 μm, in particular between 10 μm and 50 μm, particularly preferably between 15 μm and 20 μm. The length of the micro-cylindrical lenses is arbitrary, which can correspond, for example, to the total width of the thread when used in security threads and can amount to several millimeters.

In a preferred embodiment, the colored mirror has a gold or red, in particular copper, appearance in the non-open areas, whereas the colored mirror has a preferably silver appearance in the openings which are present only in the absorption layer. As already mentioned, a colored mirror can also have a plurality of different metallic reflection colors, which is achieved in particular by the presence of an absorption layer in subregions having different thicknesses.

The invention also comprises a data carrier having a security element of the type mentioned, wherein the security element is arranged in or above a window region or a through-opening of the data carrier in an advantageous configuration. The data carrier can be, in particular, a value document, such as a banknote, in particular a banknote, a polymer banknote or a film composite banknote, a stock certificate, a bond, a certificate, a coupon, a check, a high-quality ticket, but also a document, for example a credit card, a bank card, a cash card, a qualification card, an identification card or a passport identification page.

The invention also relates to a method for producing an optically variable security element of the above-mentioned type, in which method,

the relief structure and the colored mirror surface of the double layer are arranged one above the other, wherein,

The colored mirror facets are advantageously provided with openings which are present only in the ultra-thin absorption layer and form subregions of the colored mirror facets with an absorption layer having a layer thickness of zero. Alternatively or additionally, the colored mirror may be provided with openings through the absorbing layer and the metal layer.

It is particularly advantageous here to open the absorber layer and/or the entire colored mirror surface with openings by the action of the laser radiation. In particular, the openings in the ultra-thin absorber layer are opened by the action of laser radiation having a laser wavelength in which the absorption of the coloured mirror is more than 50%, preferably more than 100%, in particular more than 200%, greater than the absorption of the metal layer. In this way, a high energy introduction into the absorber layer can be achieved, which leads to selective demetallization of the absorber layer without significant damage to the metal layer. The metal layer thus exhibits almost the same reflectivity in the openings of the absorption layer as the uncoated metal layer.

Without wishing to be bound to a particular explanation, it is presently believed that the conversion of laser energy into heat occurs primarily in the absorber layer. Once the absorbing layer is demetallized, i.e. removed or converted into a transparent change, the laser radiation is reflected by the metal layer and is therefore no longer absorbed or converted into heat. The range of laser pulse energy densities that can only achieve selective demetallization of the absorber layer is therefore relatively large, which improves process stability and facilitates production on an industrial scale.

At significantly higher laser pulse energies, the entire colored mirror including the metal layer can be demetallized. That is, it can be advantageously achieved that either only the absorber layer or the entire colored mirror is demetallized selectively with the same laser source by varying the pulse energy.

Further embodiments and advantages of the invention will be explained below with reference to the drawing, in which the scaled and true-to-scale displays are omitted for the sake of increased intuitiveness.

In the drawings:

figure 1 shows a schematic representation of a banknote with an optically variable security element according to the invention,

figure 2 schematically shows a cross-section through the security element of figure 1,

FIG. 3 shows the calculated thickness d of the aluminum layer of 28nm_SiThe wavelength-dependent reflection characteristic of the two-layer colored mirror formed by the silicon layer of (a),

fig. 4 shows a further embodiment of the invention, in which the relief structure of the optically variable security element is formed by a micro-mirror structure,

fig. 5 shows a security element according to the invention, in which a relief structure in the form of a micro-mirror structure is embossed in a carrier film and a metal layer is applied,

fig. 7 shows an embodiment as in fig. 2, however with a reversed layer order of the two-layer colored mirror,

FIG. 8 shows the reflection spectrum of an aluminum layer having a thickness of 28nm and the reflection spectrum of a gold colored mirror surface constituted by a silicon layer having a thickness of 16nm on the aluminum layer having a thickness of 28nm, and

fig. 9 shows a security element according to the invention with a lens raster pattern for not-to-scale illustration of two intended given images which can be seen from different viewing directions.

The invention will now be explained by way of example of a security element for banknotes. To this end, fig. 1 shows a schematic representation of a banknote 10 having an optically variable security element 12 according to the invention. The security element 12 shows a two-color, metallic-lustrous hologram, in which a seated fox appears in this embodiment as a foreground pattern 14 in front of a gold background 16 in silver-lustrous fashion.

The security element 12 also comprises a legend 18 in the form of a serial number "20" which does not show a metallic appearance and does not participate in the reconstruction of the hologram. The legend 18 shows the underlying colour of the banknote substrate or, when the security element 12 is arranged in the window area of the banknote 10, a transparent see-through feature.

Fig. 2 shows schematically a cross section of the security element 12 in order to illustrate the basic structure of the security element according to the invention and the appearance of a two-color metallic appearance.

The security element 12 comprises a carrier film 20, for example a transparent polyethylene terephthalate (PET) film, which is provided with a relief structure 22 in the form of an embossed hologram which, when viewed, reconstructs the desired pattern, in this case a seated fox. Above the relief structure 22, a double layer of colored mirror facets 30 is arranged, which, in cooperation with the holographic relief structure 22, form a two-color optically variable appearance.

The two-layer colored mirror 30 is formed here by a reflective metal layer 32, in this example by an aluminum layer with a thickness of 28 nm. An ultra-thin absorber layer 34, which in the present example consists of a silicon layer with a thickness of 14nm, is applied on the metal layer 32. The two different reflection colors of the security element 12 result from the fact that the colored mirror 30 comprises openings in the absorber layer 34 in the partial regions 38, so that the layer thickness of the absorber layer is zero there, while the absorber layer has a nominal layer thickness of 14nm in the partial regions 36 outside the openings.

In the subregion 38 of the absorption layer 34, which has openings, the hologram 22 thus displays the silvery sparkling color of the aluminum metal layer 32 to the observer. In the partial regions 36 of the silicon layer 34, the color impression is clearly changed and the gold color is displayed, despite the small layer thickness of the silicon layer.

The two-color colored mirror 30 also comprises an opening 40 which has neither a metal layer 32 nor an absorber layer 34 and which is constructed in the embodiment of fig. 1 in the form of the reference numeral "20". In the region of the openings 40, the embossed hologram 22 is virtually invisible due to the lack of metallization, and the observer sees through the security element 12 the white or possibly printed base layer of the banknote substrate. If the security element 12 is arranged in the window region of the banknote, the opening 40 appears as a transparent see-through feature.

The color change of the metal layer 32 only occurs when the security element 12 is viewed from the side of the absorption layer 34 (viewing direction 42). Whereas a hologram pattern that is unchanged except for specularity exhibits the monochromatic silvery appearance of the aluminum metal layer 32 when viewed from the side of the metal layer 32 (viewing direction 44). In security elements 12 that are viewable from both sides, the security element can use different color impressions as additional authenticity features.

One of the main features of the present invention is the ultra-thin silicon absorber layer 34 of the two-layer colored mirror 30, which is laid on the metal layer 32. The use of such an ultra-thin absorbing layer 34 enables the reflective color of the metal layer 32 to be varied over a wide range by varying the thickness of the absorbing layer, which can produce a variety of visual effects and appearances.

For this purpose, FIG. 3 shows the calculated results for a reflective metal layer (in the form of an aluminum layer with a thickness of 28 nm) and an ultra-thin absorber layer (in the form of a thickness d)_SiIn the form of a silicon layer) of the wavelength dependent reflective properties of the two-layer colored mirror. As drawn from the line 50 corresponding to the thickness d of the silicon layer of 14nmIt can be seen that the colored mirror surface with such a silicon layer has an extremely low reflectivity in the short-wave visible spectral range and an extremely high reflectivity in the long-wave visible spectral range, so that the golden appearance described in fig. 2 results overall.

As further shown in fig. 3, the reflectivity minima of the colored mirror surface continuously shift from the blue end to the red end of the visible spectrum as the thickness of the silicon layer increases, wherein the reflectivity minima lie substantially on the reflectivity line 52. In the case of silicon layers having a very small layer thickness of a few nanometers, the silver-colored impression of the aluminum layer is predominant and the colored mirror surface appears silver-colored. If the layer thickness of the silicon layer is increased stepwise to about 30nm, the color impression of gold, yellow, red and finally blue is formed in order as the reflectivity minimum moves towards red. The color sequence is repeated starting from a layer thickness of the silicon layer of approximately 35nm, since then a steeper second reflectivity straight line 54 with a reflectivity minimum occurs, which determines the visual appearance. Due to the larger slope of the second reflectivity straight line, the reflection color of a colored mirror surface with a silicon layer having a layer thickness above 35nm is less sensitive to thickness fluctuations of the silicon layer.

Fig. 4 shows a further embodiment of the invention, in which the relief structure 62 of the optically variable security element 60 is formed by a micro-mirror structure. The security element 60 is in this case extremely flat with a maximum height difference of approximately 10 μm, but nevertheless still conveys to the observer the apparent three-dimensional impression of a curved pattern 80 in two spatial directions, which is assumed to be a spherical cap for illustration. The optically variable security element 60 comprises a reflective surface area 64, the extension of which defines an x-y plane which coincides here with the plane of the security element 60. The z-axis is perpendicular to the x-y plane, so the coordinate system consisting of these three axes constitutes a rectangular coordinate system.

Fig. 4 shows a small portion of the reflective facet area 64 having a plurality of pixels 66 disposed along the contour 84 of the curved facet 80. In the carrier film of the security element 60, the curved surface 80 perceived by the observer is not itself designed as a relief structure 62, but rather as a plurality of reflective pixels 66, each of which comprises three reflective facets 68 having the same orientation. Reflective facets 68 are small micro-mirrors that mimic the reflective properties of curved facets 80 by their orientation. The structure of facet 68 is therefore also referred to within the context of this description as a micro-mirror structure.

The reflective pixels 66 and the reflective facets 68 are formed by applying a printed lacquer layer 70 with a double-layer colored mirror 72, which consists for example of a 60nm thick aluminum layer and an 18nm thick silicon layer applied thereon and which gives the reflective surface regions 64 a colored appearance in reflection.

The orientation of each facet 68 is determined by the inclination and azimuth of the facet with respect to the x-y plane or also by the specification of its normal vector. The azimuth angle of the facet is here the angle between the projection of the normal vector n in the x-y plane and the preset reference direction. In order to imitate the reflection behavior of curved surface 80, facets 68 are each oriented such that their normal vector N corresponds exactly to local normal vector N of curved surface 80 averaged over the extent of pixel 66.

In this embodiment, the pixels 66 are designed with a square outline, but the pixels may generally have any other outline shape. The side length of the pixels 66 is less than 300 μm and in particular in the range between 20 μm and 100 μm. The facets 68 are both greater than 5 μm in length and width to avoid dispersion caused by the facet structure itself. The height of the facets is only between 0 and 10 μm, preferably between 0 and 5 μm, so that the entire reflecting surface area 64 has a height difference of at most 10 μm, which is not visible to the naked eye.

Since the geometrical reflection condition "incident angle equal to exit angle" for the reflection of the directed light 82 depends only on the local orientation of the normal vectors of the reflective faces 80, 64, and the pixel 66 is also very small and thus does not appear in appearance itself, the reflective face region 64 exhibits substantially the same reflection characteristics as the three-dimensional face 80 that needs to be mimicked, as illustrated by the leftmost pixel of fig. 4. Thus, the colored reflective surface region 64 creates a distinct three-dimensional impression of the simulated surface 80 to the viewer even with small differences in its height.

The reflective facets 68 are generally oriented such that the reflective face regions 64 perceive a protruding and/or receding surface 80 to an observer as compared to their actual spatial shape. The actual spatial shape of the reflector area 64 is given by the sequence of inclined facets, for example by a regular zigzag arrangement of facets 68 in this exemplary embodiment. Because of the generality of the described structure, colored reflective surface regions 64 can be utilized to form virtually any three-dimensionally perceptible image, such as a portrait or illustration of an article, animal, or plant, or a stereoscopic illustration of an alphanumeric character.

The reflection color of the two-layer colored mirror 30 is closely related to the layer thickness of the absorbing layer 34, as explained in connection with fig. 3 for the silicon absorbing layer. This correlation can be used to assign to the security element having the micro-mirror structure, in a single process step, different reflection colors depending on the mirror orientation. Fig. 5 shows a representation of a security element 90, in which a relief structure in the form of a micro-mirror structure 94 is embossed in a carrier film 92 and coated with a metal layer 96, for example a 60 nm-thick copper layer. As absorber layer 98, a silicon layer is vapor-deposited, wherein different layer thicknesses of absorber layer 98 are achieved by oblique evaporation coating 100 depending on the mirror slope.

If the absorption layer is at the nominal layer thickness d₀By vapor-diffusion coating, this nominal layer thickness is realized as the maximum layer thickness for the micromirror plate whose normal vector n is parallel to the evaporation coating direction 100. A micro mirror surface whose normal vector forms a small angle with the evaporation coating direction 100, for example the micro mirror surface 102 of fig. 5, is provided with a thicker, even no longer maximum thickness, absorption layer during evaporation coating, whereas a micro mirror surface whose normal vector forms a large angle close to 90 ° with the evaporation coating direction 100, for example the micro mirror surface 104, is provided with only a thinner absorption layer. More precisely, in the micro-mirror surface in which the normal vector encloses an angle β with the evaporation direction 100, the layer thickness d of the absorption layer corresponds to the nominal layer thickness d₀Is suitable for

d＝d₀cos(β)。

The color impression of a micromirror provided with an absorber layer having a layer thickness d can be determined, for example, by means of the reflection spectrum as in fig. 3.

Since oblique evaporation coating is usually achieved in production, the smaller layer thickness of the ultra-thin absorber layer shows great advantages compared to conventional colored mirror coatings with thicker dielectric layers. Therefore, significantly shorter evaporation coating times are sufficient for production, but also less material loss is achieved and the covering plate is more slowly contaminated.

The color change applied by oblique evaporation can be advantageously used, for example, in a so-called rolling star design or in oblique imaging (Kippbildern). For illustration, fig. 6 shows a detail of a security thread 110 according to the invention, which detail shows a rolling star pattern in the area of the face, wherein two mutually parallel rows of small rectangles 112 can be seen in each oblique position. A dynamic effect is created when the security thread 110 is tilted about the security thread transverse axis 114 wherein the two rectangular rows 112 move toward or away from each other. Fig. 6(a) shows the appearance in a first tilted position with a smaller rectangular row 112 distance, while fig. 6(b) shows the appearance in a second tilted position with a larger rectangular row 112 distance. The different distances of the rectangular rows 112 in the different tilted positions can be obtained in the manner shown in fig. 4 by a suitable arrangement of differently oriented facets or micro-mirrors.

The dynamic effect is combined with the color effect by oblique evaporation coating of the absorbing layer 98. Since the micro mirrors constituting the rectangular rows 112 with smaller or larger distances have different tilt angles to create different visibility in the two tilt positions, they are also provided with absorbing layers of different thickness in the above-mentioned oblique evaporation coating. In this way, the color impression of the rectangular rows 112 can be changed, for example, from a silver color impression with a small distance (fig. 6(a)) to a gold color impression with a large distance (fig. 6 (b)). The visibility and recognizability of the dynamic features can thus be further improved in a simple manner.

It should be appreciated that in practice the variation in pitch and the variation in color of the rectangular rows 112 are generally continuous, but are limited to two discrete values in fig. 6 for illustration purposes only.

In security elements with oblique images, the micro mirror can, for example, be oriented such that a first pattern is visible in a first oblique position and a second pattern is visible in a different second oblique position. Since the micro mirror surfaces have different tilt angles to create different visibility in said two tilted positions, the micro mirror surfaces are also provided with absorbing layers of different thickness in the above-mentioned tilted evaporation coating, thereby displaying, for example, a first pattern with a silver color impression and a second pattern with a red color impression similar to copper.

While in the design shown in fig. 2 first a metal layer 32 and subsequently an ultra-thin absorber layer 34 are applied to the relief structure 22, the security element 120 of fig. 7 shows that the reverse layer sequence can also be achieved. Since the color change of the metal layer 32 by the absorption layer 34 is only visible when the security element 12 is viewed from the side of the absorption layer 34, the security element 120 exhibits a constant reflection color of the metal layer 32, for example silver, when viewed from the front (in the viewing direction 122). The color impression of the metal layer 32 is changed by the ultra-thin absorption layer 34 when viewed from the rear (in the viewing direction 124), so that a color impression of, for example, gold or copper is formed.

The absorption layer 34 is left free in the partial region 38, so that the security element 120 can also show a silvery color impression there from the rear. Neither the metal layer 32 nor the absorber layer 34 is present in the opening 40, so that the opening 40 appears as a transparent see-through feature.

In another embodiment, the relief structure of the security element is formed by a two-dimensional periodic sub-wavelength grating, as described in patent 102011101635 a1, the disclosure of which is also included in the present application in this respect. The sub-wavelength grating is coated with an approximately 50nm thick layer of aluminum and a 14nm thick layer of silicon, which together constitute a two-layer colored mirror. Due to the nano-structuring, the aluminum layer, although relatively thick, is not opaque, so that the security element forms a see-through security element. The security element displays the same color when viewed from both sides and creates a different color impression in reflection according to the viewing direction, since the color change through the silicon layer is only displayed in the appearance when viewed in reflection from the side of the silicon layer.

In order to form openings only in the absorber layer or in the entire coloured mirror surface, it is possible, for example, to print washable pigments in a manner known per se and wash them off after the evaporation coating.

Particularly advantageously, however, the openings in the absorber layer or the entire colored mirror are formed by laser application, in particular by pulsed laser radiation. In the case of sufficiently high pulse energy densities, the openings 40 can be formed by demetallizing the laser radiation of the entire colored mirror, as shown in fig. 2 and 7.

Furthermore, only the absorber layer can be demetallized selectively by selecting a suitable laser wavelength without damaging the metal layer, so that the metal layer still has the desired specular reflection properties after laser loading. As mentioned above, the term demetallization includes, in addition to ablation, the conversion of the absorber layer into a transparent modification, for example by chemical conversion, such as oxidation.

The selective absorption of the absorption layer 34 is based on the observation that the two-layer colored mirror 30 as a whole has different absorption characteristics than the metal layer 32 itself. For illustration, FIG. 8 shows the reflection spectrum 130 of a 28nn thick aluminum layer and the reflection spectrum 132 of a gold colored mirror 30 consisting of a 16nm thick silicon layer on a 28nm thick aluminum layer. As shown in fig. 8, the colored mirror 30 exhibits significantly reduced reflection at wavelengths below about 650nm as compared to the pure aluminum layer 32, which shows a golden appearance to the human eye. Since the transmission of the aluminum layer 32 and the colored mirror 30 is negligibly low, the unreflected radiation energy is absorbed separately. By choosing a laser wavelength at which the coloured mirror 30 absorbs strongly and the aluminium layer 32 absorbs little, a high heat introduction into the absorbing layer can be achieved, which leads to selective demetallisation of the absorbing layer 34, but without significant damage to the aluminium layer 32.

For example, only the 16nm thick silicon layer 34 of the gold colored mirror 30 can be selectively demetallized by pulsed laser loading with laser radiation from a double-frequency Nd: YAG laser with a wavelength of 532 nm. The demetallized sub-regions show the original silvery appearance and specular reflection of the aluminium layer 32, which means that the surface of the aluminium layer 32 is substantially undamaged. At significantly higher pulse energies, the entire colored mirror 30, along with the aluminum layer 32, can be demetallized.

In other embodiments, the relief structure of the optically variable security element can also be formed by a microlens grating, in the focal plane of which a double-layer colored mirror is arranged. For illustration, fig. 9 shows a security element 140 with a lenticular image to show, not to scale, two predetermined given images, which are visible from

different viewing directions

142, 144.

The security element 140 has a carrier 150 in the form of a transparent plastic film, for example a PET film about 20 μm thick. The upper side of the carrier 150 is provided with a viewing grid in the form of a plurality of parallel cylindrical lenses 152, the width b of which is 20 μm in this embodiment. A pattern layer 154 is provided on the underside of the carrier 150, wherein the thickness of the carrier 150 and the curvature of the microlenses are matched to one another such that the focal length of the microlenses 152 corresponds substantially to the thickness of the carrier 150, so that the pattern layer 154 lies in the focal plane of the microlenses 152.

The patterned layer 154 is formed in this exemplary embodiment by a colored, for example blue, lacquer layer 156 and a two-layer colored mirror 158, which is arranged on the lacquer layer 156 and has a metal layer 160 and an ultra-thin absorption layer 162. For example, metal layer 160 refers to a 28nm thick layer of aluminum and ultra-thin absorber layer 162 refers to a 14nm thick layer of silicon, so the colored mirror appears golden in its unopened areas.

To form a given image, a plurality of

micro-holes

164, 166 are provided in the colored mirror 158 by the action of laser radiation. The first micropores 164 are created from the first viewing direction 142 only in the ultra-thin absorber layer 162 in the manner described above. At higher pulse energies from second viewing direction 144, second micro-pores 166 are created that extend through the entire colored mirror 158. The first plurality of microholes 164 collectively form a first given image and the second plurality of microholes 166 form a second given image. Due to the reversibility of the beam path, it is ensured that a given image which is provided in the colored mirror 158 by means of laser loading is then visible when viewed from the

same direction

142, 144, respectively, as the direction in which the given image is irradiated by the laser beam.

Since the first given image is formed by micropores 164 present only in the ultra-thin absorber layer 162, the first given image appears silver in color before the gold background of the non-apertured colored mirror region. The second given image is formed by the micro-holes 166 through the entire colored mirror 158, so that the blue paint layer 156 is visible there, and the second given image is blue in front of the gold background of the non-apertured colored mirror region.

It will be understood that the security element shown in the figures generally comprises further layers, for example protective layers, covering layers or other functional layers, which are however not essential for the invention and are therefore not described further.

List of reference numerals

10 banknote

12 Security element

14 foreground pattern

16 background

18 yin language

20 carrier film

22 relief structure

30 colored mirror

32 metal layer

34 ultra-thin absorption layer

36. 38 sub-region

40 open pores

42. 44 viewing direction

Line with thickness d of 50 silicon layer being 14nm

52. 54 straight reflection line

60 security element

62 relief structure

64 area of reflecting surface

66 reflective pixel

68 reflecting facets

70 printing paint layer

72 colored mirror

80 curved pattern

82 directed light

84 contour line

90 security element

92 carrier film

94 micro mirror device

96 metal layer

98 absorption layer

100 oblique evaporation coating

102. 104 micro mirror surface

110 security thread

112 rectangle

114 line transverse axis

120 security element

122. 124 viewing direction

130. 132 reflectance spectrum

140 security element

142. 144 direction of observation

150 vector

152 microlens

154 pattern layer

156 colored paint layer

158 colored mirror

160 metal layer

162 absorbing layer

164. 166 micro-hole

Claims

1. An optically variable security element for providing security to an item of value, wherein,

the relief structure and the coloured mirror surface of the double layer are arranged one above the other and, under the combined action, form an optically variable and coloured appearance in reflection, wherein,

the colored mirror surface of the bilayer is composed of a reflective metal layer and an ultra-thin absorbing layer arranged on the metal layer,the ultrathin absorption layer is made of silicon, silicon alloy or SiO with x less than 1_xWherein the ultra-thin absorption layer has a layer thickness between 1nm and 200 nm.

2. A security element according to claim 1, characterized in that the colored mirror comprises at least two sub-areas having different layer thicknesses of the ultra-thin absorbing layer, including a layer thickness of zero, and the colored mirror has different reflection colors in the at least two sub-areas, so that the relief structure and the colored mirror of the double layer form under co-action an appearance of at least two colors in reflection.

3. A security element according to claim 1, characterized in that the ultra-thin absorbing layer has a layer thickness between 1nm and 100 nm.

4. A security element according to claim 1, characterized in that the ultra-thin absorbing layer has a layer thickness between 5nm and 35 nm.

5. A security element according to claim 1, characterized in that the metal layer has a layer thickness of between 10nm and 100 nm.

6. A security element according to claim 5, characterized in that the metal layer has a layer thickness of between 15nm and 80 nm.

7. A security element according to claim 1, characterized in that the colored mirror comprises openings which are present only in the ultra-thin absorbing layer, said openings constituting sub-areas of the colored mirror with the ultra-thin absorbing layer having a layer thickness of zero.

8. A security element as claimed in claim 1 wherein the coloured mirror comprises openings through the ultra-thin absorbing layer and the metallic layer.

9. A security element according to claim 8, characterized in that a coloured mirror is combined with a coloured layer, which is visible in the openings of the coloured mirror.

10. A security element according to claim 9, characterized in that the colored layer is present on the side of the metal layer facing away from the ultra-thin absorbing layer.

11. A security element according to claim 1, characterized in that the relief structure is a diffractive structure, an achromatic structure or a nanostructure.

12. A security element as claimed in claim 11, characterized in that the diffractive structure is a hologram, a holographic grating image or a hologram-like diffractive structure.

13. A security element according to claim 11, wherein the achromatic structure is a matt structure, a micro-mirror device, a blazed grating with a sawtooth-like groove profile, or a fresnel lens device.

14. A security element according to claim 11, wherein the nanostructures are subwavelength structures.

15. A security element according to claim 1, characterized in that the colored mirror surface of the bilayer is designed as a relief structured coating.

16. A security element according to claim 1, wherein the relief structure is a lenticular lens consisting of a plurality of lenticules arranged in spaced relation to the coloured mirror, and wherein the coloured mirror comprises one or more given images which are recognizable from certain viewing directions when the coloured mirror is viewed through the lenticular lens.

17. A security element according to claim 16, wherein a given image comprises openings only in the ultra-thin absorbing layer and/or openings in the entire coloured mirror surface.

18. A security element according to claim 7, wherein the coloured mirror has a gold or red appearance in the non-apertured areas.

19. A security element as claimed in claim 7 wherein the coloured mirror has a copper-coloured appearance in the non-apertured areas.

20. A security element according to claim 7, wherein the coloured mirror also has a silvery appearance in the openings only present in the ultra-thin absorbing layer.

21. A data carrier having a security element according to any one of claims 1 to 20.

22. A method for producing an optically variable security element according to one of claims 1 to 20, in which method,

the double-layer colored mirror consists of a reflective metal layer and an ultrathin absorption layer arranged on the metal layer, the ultrathin absorption layer consisting of silicon, a silicon alloy or SiO with x < 1_xWherein the ultra-thin absorption layer has a layer thickness between 1nm and 200 nm.

23. A method as claimed in claim 22, characterized in that the colored mirror is provided with openings which are present only in the ultra-thin absorption layer and constitute sub-areas of the colored mirror with an ultra-thin absorption layer having a layer thickness of zero.

24. A method as claimed in claim 22 or 23, characterized in that the colored mirror is provided with openings which extend through the ultra-thin absorption layer and the metal layer.

25. A method as claimed in claim 23, characterized in that the openings are opened only in the ultra-thin absorption layer and/or in the entire coloured mirror surface by the action of the laser radiation.

26. The method of claim 24 wherein the openings in the ultra-thin absorber layer are created by the action of laser radiation having a laser wavelength at which the absorption of the colored mirror is greater than the absorption of the metal layer by more than 50%.

27. The method of claim 26 wherein the absorption of the colored mirror at the laser wavelength is greater than the absorption of the metal layer by more than 100%.

28. The method of claim 26 wherein the absorption of the colored mirror at the laser wavelength is greater than the absorption of the metal layer by more than 200%.