The invention relates to a security element for manufacturing value documents, such as bank notes, checks or the like, that has a substrate with an upper side and provides at least one image, wherein the image is formed by a grating structure which has a plurality of microcavities, and the microcavities respectively have in a spatial direction lying parallel to the upper side a structure width from 0.5 μm to 3 μm and perpendicular thereto a structure depth and have an aspect ratio which is defined by the ratio of structure depth to structure width, and a metal-containing coating applied to the grating structure.
The invention also relates to a value document with such a security element.
Further, the invention relates to a manufacturing method for a security element for value documents, such as bank notes, checks or the like, which provides at least one image, wherein for generating an image a grating structure having a plurality of microcavities side by side is formed on a substrate which has an upper side, the microcavities respectively are formed in a spatial direction lying parallel to the upper side, with a structure width of 0.5 μm to 3 μm and perpendicular thereto with a structure depth, and have an aspect ratio which is defined by the ratio of structure depth to structure width, and on the grating structure a metal-containing coating is applied.
Security elements serve to make it more difficult to copy or forge value documents such as bank notes, checks or the like. For such security elements, embossed structures have proved themselves which have, for example, microcavities in the form of a grating structure.
In the prior art security elements which have a microcavity structure are known. They provide microimages which are visible when enlarged with moiré magnification arrangements. The design of microimages for moiré magnification arrangements is discussed already in many respects in the prior art.
Also known are microcavity structures to effectuate an absorption of incident radiation and thereby, for example, supply a background for an image. The EP 1434695 B1 describes an absorbent structure with a period smaller than the light wavelength. The structure is constructed as a cross grating with sinusoidal profile. The WO 2005/106601 A2 relates to moiré magnification arrangements with microimages which consist of anti-reflective regions and partly reflective regions. The anti-reflective area is formed by nanostructures with a period smaller than 700 nm and a depth between 150 nm and 350 nm. In the EP 1979768 A1, multilayer bodies with microlens arrangement are explained in which microimages are generated by microholes or regions having different opacity. The WO 2002/101669 A2 describes microimages which are formed by fine points or perforations. The EP 1476317 A1 as well as the U.S. Pat. No. 7,468,842 B2 describe concave or convex surfaces, image elements as relief surfaces, which are filled with color, and “light trap patterns” formed through subwavelength structures, for producing microimages for moiré magnification arrangements.
From the DE 102008046128 A1 a matt structure is known for a security element which has a plurality of microelements with respectively a lateral dimension under 50 μm, wherein at least one geometry parameter of the microelements varies randomly to generate the matt effect. The geometry parameter can be the depth of the microelements.
The WO 2005/095119 A1 describes a security element in form of a multilayer foil body which shows a color alteration effect by means of interference dependent on angle of view. A relief structure, which is so configured with respect to its geometry that the color alteration effect is generated only in individual sections, is thereby molded in a replication lacquer layer. For this purpose the aspect ratio of the relief structure is varied.
The DE 102006050047 A1 describes a transmission view security element with microstructures, which are configured as microcavities which have a distance of 1 μm from each other.
Metallized security elements which have an elevated transmission in finely structured regions are known from the EP 1786632 B1. Therein a relief structure furnished with a metal layer is disclosed which is configured as a diffractive structure and contains regions of sinusoidal linear gratings or of cross gratings which have a period in the subwavelength region. To achieve the desired transmission effect, the structure elements have an aspect ratio of greater than 0.5. Such structures are also known from the DE 10 2004 042 136 A1. Manufacturing such structures, in particular also replicating and embossing subwavelength structures with high aspect ratio, is not quite unproblematic.
From the WO 2012/069163 A1, a reflective security element is known, which has a microcavity structure, which works as retroreflectors. A colored image is produced in plan view through suitable structuring.
Furthermore holograms are employed for security elements. Conventional holograms are relief structures vaporized with aluminum. The relief structures form a diffraction grating with different periods and orientations. A viewer perceives the hologram effect in the first diffraction order of the reflection. The grating profile of known embossed holograms has as a rule a sinusoidal cross section, as found in the U.S. Pat. No. 7,129,028, for example. Such embossed holograms appear uncolored in the reflection and are dark in transmitted light, i.e. have substantially no transmission.
As security elements are further known grating structures which show a color effect in the zeroth diffraction order in transmission. The WO 2012/019226 A1 describes so-called zero-order gratings which are formed by color filling or nanoparticles in an emboss structure. Upon transmissive viewing, color and polarization depend on the viewing angle.
The WO 2013/053435 A1 describes a rectangular grating with metallized plateaus which lie at different height levels and have a thin metal layer. Such structures have an angular-dependent color filtering upon transmitted light. However, they show no distinctly perceptible first diffraction order in the reflection, because the grating period lies in the subwavelength region.