CA2993901A1

CA2993901A1 - Security element having a subwavelength grating

Info

Publication number: CA2993901A1
Application number: CA2993901A
Authority: CA
Inventors: Hans Lochbihler
Original assignee: Giesecke and Devrient Currency Technology GmbH
Current assignee: Giesecke and Devrient Currency Technology GmbH
Priority date: 2015-08-06
Filing date: 2016-07-25
Publication date: 2017-02-09
Also published as: DE102015010191A1; CN107949484A; EP3331709A1; CN107949484B; KR102511203B1; KR20180037970A; WO2017021000A1; EP3331709B1; JP2018528486A

Abstract

The invention relates to a security element for producing documents of value, such as banknotes, checks or the like, which has: a dielectric substrate (1), a first periodic line grating structure (2), embedded in the substrate (1), made of a plurality of first grating webs (3) running along a longitudinal direction and arranged in a first plane (L1) having first grating gaps (4) located in between, and a second line grating structure (6) of the same period (d) embedded in the substrate (1) and made of second grating webs (7) running along the longitudinal direction and having second grating gaps (8) located in between, wherein the second line grating structure (6), in relation to the first plane (L1), is located above the first line grating structure (2) in a parallel, second plane (L2), and wherein the second line grating structure (6) is formed so as to be inverted with respect to the first line grating structure (2) such that, in plan view of the first plane (L1), the second grating webs (7) lie above the first grating gaps (4) and the second grating gaps (8) lie above the first grating webs (3), wherein the security element (S) produces a color effect when viewed in transmission (T), and the grating webs (3) of the first line grating structure (2) and the grating webs (7) of the second line grating structure (6) are each formed from a double layer, built up from a layer of highly refractive material (3a, 7a) and a layer of metallic material (3b, 7b).

Description

SECURITY ELEMENT HAVING A
SUBWAVELENGTH GRATING
The invention relates to a security element for producing valuable documents such as banknotes, checks or the like, the security element comprising: a dielectric substrate, a first periodic line grating structure that is embedded in the substrate and made of a plurality of first grating webs, extending in a longitudinal direction and arranged in a first plane, which have first grating gaps located therebetween, and a second line grating structure of the same period that is embedded in the substrate and made of second grating webs, extending in the longitudinal direc-tion, which have second grating gaps located therebetween, wherein the second line grating structure is located, relative to the first plane, above the first line grat-ing structure in a parallel, second plane and wherein the second line grating struc-ture is formed to be inverted with respect to the first line grating structure such that, in top view onto the first plane, the second grating webs are located above the first grating gaps and the second grating gaps are located above the first grat-ing webs.
Security elements having periodic line gratings are known, for example from DE
102009012299 Al, DE 102009012300 Al or DE 102009056933 Al. They can have color filter properties in the subwavelength range if the grating profile is such that resonance effects occur in the visible spectral range. Such color filter properties are known both for reflective and for transmissive subwavelength structures.
Said structures have a strongly polarizing influence on the reflection or transmission of an incident light beam. The color is relatively strongly angle-dependent in reflec-tion or transmission of such subwavelength gratings. However, the color satura-tion for these gratings diminishes considerably if the incident light is unpolarized.

- 2 -One-dimensionally periodic gratings can have color filter properties in the sub-wavelength range if the grating profile is such that resonance effects occur in the visible wavelength range. Said color filter properties depend on the angle of the incident light.
DE 3248899 C2 describes a subwavelength structure that has angle-dependent color-filtering properties. Said grating has a rectangular cross section and has de-posited on it a high refractive (HRI) layer, wherein for the refractive indexes:
nFiRi>n2 and n1n2n3. A color change occurs upon variation of the angle O. If the grating is tilted perpendicularly with respect to the plane of incidence (0>0 ;
(D=900), the color remains approximately constant. The angle (to designates the azi-muth angle. The security element sold under the name DID ("Diffractive Identifi-cation Device") is based on said structure and uses the color filter properties in re-flection. Required is a light-absorption substrate so as to perceive a color effect.
WO 2012/019226 Al describes embossed subwavelength gratings likewise having a rectangular profile, printed on the plateaus of which are metal particles or me-tallic nanoparticles. Said grating exhibits color effects or polarization effects in transmission.
Also known are subwavelength gratings as angle-dependent color filters that have a metallic or semimetallic double layer arrangement, e.g. from DE
102011115589 Al or from Z. Ye et al., "Compact Color Filter and Polarizer of Dou-ble layer Metallic Nanowire Grating Based on Surface Plasmon Resonances," Plas-monics, 8, 555-559 (2012), in which the metalization is realized by way of vapor deposition and is embedded in a dielectric. The approach described in DE
102011115589 Al, which discloses a security element having the above-mentioned features, is based on an arrangement of two wire gratings having the same period,

- 3 -which are offset relative to one another by half a period and consist of metallic or semimetallic (e.g. ZnS with a thickness of 70 nm) wires.
This known subwavelength structure having a ZnS coating of approximately 70 nm thickness is suitable as a color filter in reflection. The structure must therefore additionally be applied onto a light-absorption substrate so as to achieve a suffi-cient color contrast that is then visible in reflection. Subwave gratings having me-tallic coatings exhibit a relatively high color saturation in transmission.
Due to the light absorption in the metal, they therefore appear relatively dark.
Sinus gratings coated with a thin metal film can cause plasmonic resonance ef-fects. Said resonances result in increased transmission in TM polarization, cf. Y.
Jourlin et al., "Spatially and polarization resolved plasmon mediated transmission through continuous metal films"; Opt. Express 17, 12155-12166 (2009). This effect can be optimized further by an additional thin dielectric layer, such as disclosed by T. Tenev et al., "High Plasmonic Resonant Reflection and Transmission at Con-tinuous Metal Films on Undulated Photosensitive Polymer," Plasmonics (2013).
The security element described in WO 2012/136777 Al is based on this optical ef-fect.

4 A2 likewise describes transmissive security elements that are based on subwavelength gratings and exhibit an angle-dependent color. The opti-cal properties of sinus gratings coated in a highly refractive manner are discussed in more detail in said document.
The known two-dimensional periodic subwavelength gratings with non-contigu-ous surface exhibit color filter properties, but have a high angle tolerance.
The hue thereof therefore hardly changes upon tilting.

The invention is therefore based on the object of specifying a security element that exhibits on inspection a good color effect which changes upon tilting.
This object is achieved in accordance with the invention by way of a security ele-ment for producing valuable documents such as banknotes, checks or the like, the security element comprising:
- a dielectric substrate, - a first periodic line grating structure that is embedded in the substrate and made of a plurality of first grating webs, extending in a longitudinal direction and arranged in a first plane, which have first grating gaps located therebetween, and - a second line grating structure of the same period that is embedded in the substrate and made of second grating webs, extending in the longitudinal direc-tion, which have second grating gaps located therebetween, - wherein the second line grating structure is located, relative to the first plane, above the first line grating structure in a parallel, second plane and - wherein the second line grating structure is formed to be inverted with re-spect to the first line grating structure such that, in plan view of the first plane, the second grating webs are located above the first grating gaps and the second grat-ing gaps are located above the first grating webs, wherein - the security element produces a color effect in transmission observation and

-5-- the grating webs of the first line grating structure and the grating webs of the second line grating structure are formed each from a double layer, made of a layer of a highly refractive material and a layer of a metallic material.
The highly refractive material is preferably dielectric or a semiconductor, e.g. Si, Ge, C.
According to the invention, a double line grating is used consisting of line grating structures that are located one above the other in two planes in a mutually corn-plementary fashion, i.e. offset with respect to one another. A phase shift of 900 is the ideal value, which must of course be considered in the context of manufactur-ing accuracy. Due to manufacturing tolerances, deviations from the complementa-rity, i.e. 90 phase shift, can occur here. In addition, a rectangular profile may not be formed perfectly, but approximated by way of a trapezoidal profile, with the upper parallel edge thereof being shorter than the lower one. In the case of a line grating structure with a rectangular cross section, the phase shift corresponds to half a period.
The line grating structures are made up of a combination of a layer of highly re-fractive, dielectric or semimetallic material with a metallic layer. The thickness of the grating webs is less than the modulation depth, i.e. than the spacing of the planes of the line grating structures, with the result that no closed film is pro-duced. For this reason, the spacing of the first and second planes is greater than the sum of (0.5 * first layer thickness) and (0.5 * second layer thickness).
It was found that a grating of such a structure surprisingly provides, in transmis-sion observation, reproducible and easily perceivable color effects upon tilting.

- 6 -The security element can be produced simply as a layer structure, by first provid-ing a base layer on which the double layer of the first line grating structure is formed. Applied thereon is a dielectric intermediate layer that covers the first line grating structure and is thicker than the grating webs of the first line grating structure. Formed thereon can then be the offset, second line grating structure, and a dielectric cover layer completes the substrate in which the line grating struc-ture is embedded. Alternatively, it is also possible first for a subwave grating to be formed, e.g. embossed, in the dielectric substrate, which subwave grating has a rectangular profile in cross section. If it is perpendicularly coated, e.g. by vapor deposition, with the materials of the double layer, the double layer is produced on the plateaus and in the trenches, which forms the first and second grating webs.
As a result, the desired first and second grating webs are obtained in different planes.
A particularly good color effect is achieved if the vertical spacing between the first and the second grating webs, i.e. the modulation depth of the structure, is be-tween 100 nm and 500 nm. The measurement of the spacing is based on the two planes that can be defined e.g. by identically directed surfaces of the first and sec-ond line grating structures, i.e. for example on the lower side of the grating webs or the top side of the grating webs. The vertical spacing should here of course be measured perpendicularly to the plane, i.e. it designates the height difference be-tween identically directed surfaces of the grating webs.
Suitable highly refractive materials for the double layer of the grating webs are all materials that have a higher refractive index, in particular by at least 0.3, than the surrounding substrate, i.e. material. The layer sequence in the double layer is ir-relevant; it can also differ for the first and the second line grating structure.

- 7 -The security element exhibits angle-dependent color filtering upon observation in transmission. This angle dependence is particularly marked if the grating lines are perpendicular with respect to the plane of light incidence. Color filtering can be used to produce motifs in multiple colors such that they change their color in de-pendence on the rotational position or exhibit different effects upon tilting of the plane. It is therefore preferred for at least two regions to be provided in plan view of the plane, of which the longitudinal directions of the line grating structures are at an angle with respect to one another, in particular at a right angle. Upon per-pendicular observation, a motif can be produced such that it has a uniform color and no further structure upon perpendicular observation. If this security element is tilted, the color of one region, for example of the background, changes in a man-ner that is different from the color of the other region, for example of a motif.
Also conceivable of course are embodiments having a plurality of differently ar-ranged regions. For example, one development is provided that has a plurality of regions in the security element, wherein the regions differ from one another with respect to the orientation of the grating lines and/or grating period of the line grating structures. As a result, motifs with different color effects in transmission observation can be provided.
It is to be understood that the previously mentioned features and the features yet to be explained can be used not only in the specified combinations, but also in dif-ferent combinations or alone, without departing from the scope of the present in-vention.
The invention will be explained in more detail below for example on the basis of the attached drawings that also disclose features essential to the invention.
In the figures:

- 8 -Figure 1 shows a sectional view of a security element having a double line grat-ing, with each line grating having grating webs from a double layer, Figures 2a-b show the spectral dependence of the reflectance and transmittance of the security element in figure 1 upon variation of the observation angle, Figures 3a-b show the spectral dependence of the reflectance and transmittance of the security element in figure 1 upon variation of the modulation depth h, Figures 4a-b show the spectral dependence of the reflectance and transmittance of the security element in figure 1 upon variation of the modulation depth and for a different material combination than in figures 3a-b, Figure 5 shows color values in the LCh color space for reflectance and transmit-tance for the security element in figure 1 upon variation of a layer thickness in the double layer and for different observation angles, Figures 6a-b show a CIE 1931 color diagram for reflectance and transmittance of the security element in figure 1 upon variation of a layer thickness, Figures 7a-b show a CIE 1931 color diagram for reflectance and transmittance of the security element in figure 1 upon variation of the observation angle and for different layer thicknesses than in figures 6a-b, Figures 8a-b show a CIE 1931 color diagram for reflectance and transmittance of the security element in figure 1 upon variation of the observation angle and for different layer thicknesses than in figures 7a-b,

- 9 -Figures 9a-b show two plan views of a security element that is configured as a motif with gratings of figure 1, but with different orientations of the gratings, and Figures 10a-b show illustrations similar to figures 9a-b for a further embodiment of the security element.
Figure 1 shows, in a sectional view, a security element S having a double line grat-ing that is embedded in a substrate and consists of two line grating structures 2, 6.
The substrate comprises a dielectric carrier 1, on which a first line grating struc-ture 2, arranged in a plane L1, is incorporated in a dielectric layer, e.g. an emboss-ing lacquer layer. The first line grating structure 2 consists of first grating webs 3 having a width b, which extend in a longitudinal direction that is perpendicular to the drawing plane. Located between the first grating webs 3 are first grating gaps 4, having a width a.
Each grating web 3 consists of a double layer of highly refractive material 3a hav-ing a thickness t4 and metallic material 3b having a thickness t2. The thickness of the first grating webs 3 (measured perpendicularly to the plane L1) is thus t2 + t4.
Located at a height h above the first grating webs 3 is, in a plane L2, a second line grating structure 6 having second grating webs 7, likewise made of the double layer of highly refractive material 7a having a thickness t3 and metallic material 7b having a thickness t1. The second grating webs 7 have the width a. The second line grating structure 6 is phase-shifted in the plane L2 with respect to the first line grating structure 2 such that the second grating webs 7 come to lie as exactly as possible (in the context of manufacturing accuracy) above the first grating gaps 4. At the same time, second grating gaps 8, located between the second grating webs 7, are situated above the first grating webs 3.

- 10 -The thickness t2 + t4 of the first grating webs 3 is less than the height h, with the result that no contiguous film is formed from the grating webs 3 and 7. The height h represents a modulation depth of the grating structures.
In the schematic sectional view in figure 1, the width b of the first grating webs 3 is identical to the width a of the second grating webs 7. With reference to a period d, the fill factor in each line grating structure is thus 50%. However, this is not mandatory. In accordance with the formula b + a = d, any desired variation can take place.
In addition, in the schematic sectional view of figure 1, the thicknesses are t2 = t4 and t1 = t3 and also (t2 + t4) = (t1 + t3). This is beneficial for simpler production, but not absolutely mandatory.
The modulation depth h, i.e. the difference in height between the first line grating structure 2 and the second line grating structure 6 (corresponding to the spacing of planes L1 and L2), is greater than the sum of the thicknesses of the first grating webs 3 and the second grating webs 7, with the result that a vertical spacing with the dimension h-(t2+t4) is present between the two line grating structures 2 and 6.
The grating structure can be considered to be an arrangement of two wire grat-ings having the same profile, which are located at a spacing h-(t2+t4) with respect to one another.
The grating webs 3, 7 in all embodiments are formed from a double layer of a highly refractive, dielectric or semimetallic material 3a, 7a and a metallic material 3b, 7b. The highly refractive material has the index of refraction num and is sur-rounded by dielectrics, specifically a dielectric intermediate layer 5 and a dielec-tric cover layer 10. In practice, the indices of refraction of these surrounding mate-rials generally hardly differ and approximate fli. The index of refraction num of

- 11 -the highly refractive material is higher than that! those of the surrounding mate-rial, e.g. by at least 0.3 in absolute terms.
The security element S of figure 1 reflects incident radiation E in the form of re-flected radiation R. Furthermore, a radiation component is transmitted in the form of transmitted radiation T. The reflection and transmission properties depend on the angle of incidence 0, as will be explained below.
The security element S can, for example, be produced by applying first the first line grating structure 2 on the carrier 1 and then applying the intermediate layer 5 on the former. The second line grating structure having the second grating webs 7 can then be introduced into the grating gaps 4 which are here shown upward. A
cover layer 10 covers the security element. The indices of refraction of the layers 5 and 10 and of the carrier 1 are substantially identical in some embodiments and can be, for example, ni=1.5, in particular 1.56.
The dimensions b, a and t1 to t4 lie in the subwavelength range, i.e. are less than 300 nm. The modulation depth h is preferably between 100 nm and 500 nm.
Also possible is a manufacturing method in which, first, a rectangular grating is produced on a top side of the carrier 1. In other words, the carrier 1 is structured such that trenches having the width a alternate with webs having the width b.
The structured substrate is subsequently provided with the desired coating by way of vapor deposition, such that the first and second line gratings and the first and sec-ond line grating structures are formed. After vapor deposition, the structure is fi-nally covered with a cover layer. This gives a layer structure in which the upper and lower sides substantially have the same refractive index.

- 12 -The structured substrate can be obtained in different ways. One option is the re-production using a master. The master mold can be replicated e.g. in UV
lacquer on a film, e.g. PET film. This gives the substrate 1 as a dielectric material, which has, for example, an index of refraction of 1.56. Alternatively, hot embossing methods can be used.
The master or the substrate itself can be produced using an e-beam installation, a focused ion beam or using interference lithography, with the structure being writ-ten into a photoresist and subsequently being developed.
The structure of a master produced by way of photolithography can in a subse-quent step be etched into a quartz substrate so as to form flanks of the profile that are as perpendicular as possible. The quartz wafer is then used as a preform and can be copied, e.g. in Ormocer, or be replicated by way of galvanic forming.
Direct forming of the photolithographically produced original in Ormocer or in nickel in a galvanic method is likewise possible. It is also possible to compose a motif with different grating structures in a nanoimprint method starting from a homogene-ous grating master.
Such manufacturing methods for subwavelength grating structures and for motifs consisting of different subwavelength structures are known to a person skilled in the art, e.g. from DE 102011115589 Al, which in this respect is incorporated here in its entirety.
The optical properties of the security element will be discussed below for alumi-num and the highly refractive materials zinc sulfide (ZnS) and titanium dioxide (Ti02) in the visible wavelength range by way of example. The surrounding mate-rial is polymer having a refractive index of n=1.52. It is furthermore assumed that the profile geometry of the grating webs is rectangular. Minor deviations from

- 13 -this ideal rectangular shape that occur in practice, such as e.g. a trapezoidal shape, do not majorly influence the optical appearance and give similar results as for rec-tangular gratings. Figures 2a and 2b show the spectral reflectance (figure 2a) and the transmittance (figure 2b) for a grating having the parameters d = 360 nm, h =
220 nm, b = 180 nm, and the coatings tm = 30 nm and tzns = 160 nm. The incident light is unpolarized.
Figure 2a shows on the y-axis the reflectance as a function of the wavelength plot-ted on the x-axis for different angles of incidence, specifically 00, 15 and 300. Fig-ure 2b analogously shows the transmittance. The angle of incidence 0 is defined in figure 1.
The spectral reflectance shows, for normal incidence of light, two significant dips at 404 nm and at 672 nm, wherein the long-wave dip can be found as a peak in the transmittance spectrum. For an increasing angle of incidence, this peak shifts into the long-wave range, and further peaks occur in the transmittance spectrum that have an angle-depending dispersion.
Figures 3a and 3b relate to the influence of the modulation depth h on the trans-mittance spectrum. The figure shows the transmittance as a function of the wave-length in the visible range for a grating having the coatings tm = 30 nm and tzns =
140 nm for a normal incidence of light (figure 3a) and for the angle of incidence 0 = 30 (figure 3b). The modulation depth is varied between 180 nm and 240 nm.
For normal incidence, three peaks can be seen, wherein the two short-wave peaks are significantly influenced in terms of their markedness by the variation of the modulation depth. The intensity in the blue peak increases strongly and shifts into the green, while the intensity of the peak at the wavelength 560 nm strongly de-creases. For the angle of incidence 0 = 30 , the location of the peaks in the visible range is nearly unchanged when the modulation depth h is varied. The grating

- 14 -has the parameters d = 360 nm, b = 180 nm, and the coating tied = 30 nm and tzns =
140 nm, embedded in a dielectric with n=1.52 and modulation depths h = 180 nm - 240 nm.
Figures 4a and 4b relate to the influence of the highly refractive material on the diffraction behavior of the grating. The figure shows the transmittance spectra of a grating with the parameters of figure 3, but a coating, having a thickness of 140 nm, with TiO2 instead of ZnS. The blue component in the spectrum is here signifi-cantly higher, because TiO2 has a significantly lower absorption in the blue range.
In addition, the transmittance in the red range is higher as a whole. The resonance in this wavelength range for e = 30 is more weakly marked, which also results in a lower light absorption.
To examine the color properties of said security elements in the LCh color space, a convolution of the transmittance and reflectance spectra with the emission curve of a D65 standard lamp and the sensitivity of the human eye was performed and the color coordinates X, Y, Z were calculated. D65 illumination approximately cor-responds to daylight. The XYZ coordinates were subsequently converted into the color values LCh. These values can be directly associated with human sensation in the color perception of an observer:
L*: lightness, C*: chroma (= color saturation), and h : hue.
Figure 5 shows the LCh color diagrams of a security element (in reflection on the left and in transmission on the right) with the parameters d = 360 nm, h = 210 nm, b = 180 nm as a function of the thickness t3 = t4 of the ZnS coating 3a, 7a for the angles of incidence e = 0 and 30 . What can be seen here is that a ZnS layer thick-ness of approximately 160 nm produces a particularly strong change in the

- 15 -chroma in transmission during tilting, i.e. a change in the angle O. The change in hue, on the other hand, increases for increasing thicknesses.
The values of figure 5 were converted into x, y color coordinates and are shown in figures 6a, b in the CIE 1931 color space. The white point is designated "WP."
The triangle delimits the color region which can typically be represented using screens. The color coordinates are illustrated in the diagram in the form of trajec-tories. The endpoint of the thickness tzns = 200 nm is designated with a dot-shaped symbol. In reflection, the hue changes due to the variation of the layer thickness of ZnS. During tilting from 00 to 30 , the color changes between yellow and red. In transmission, on the other hand, a relatively large region of the color space is covered by the variation of the ZnS thickness. While an aluminum grating in accordance with DE 102011115589 Al without additional ZnS coating would show a color shift effect from yellow to magenta, these colors in a grating that has a 180 nm ZnS coating virtually appear in the inverse order.
The color properties of the reflection are illustrated in figure 7a and the color dia-gram of the transmission is illustrated in figure 7h for a security element having the parameters d = 360 nm, h = 220 nm, b = 180 nm and the layer thicknesses tm =
30 nm and tzns = 160 nm, embedded in a dielectric with n=1.52. Figures 7a, b show the CIE 1931 color diagrams in reflection (figure 7a) and in transmission (figure 7b), in which the color coordination is plotted as a function of the angle of inci-dence 0 from 00 to 30 . Here, the illumination of the security element having the layer sequence of figure 1 and a security element with the inverse layer sequence, equivalent to the illumination from the reverse, was examined. The trajectory of the illumination of the front is designated "V," the associated trajectory of the rear illumination is characterized "R." It should be pointed out that the transmission for these two cases is identical due to the reciprocity of the light path. In transmis-sion, a marked color shift effect from blue to green occurs. In reflection, the color

- 16 -change is significantly weaker. However, the reflected color of the front is clearly distinguishable from that of the rear. This effect additionally increases the anti-forgery protection when using such grating structures as a security feature.
Figures 8a and 8b show the x, y color coordinates of a security element similar to that of figures 7a and 7b, but with different grating parameters. The data is like-wise shown in color diagrams for reflection and transmission as a function of the angle of incidence 0 = 00-300. In contrast to figures 7a and 7b, the grating period is here d = 320 nm. The ratio b/d is likewise 0.5. The layer thicknesses are tm =

nm and tzns = 120 nm. In reflection, the green hue hardly changes upon tilting. It primarily varies the color saturation. However, in transmission the hue changes from red to blue with a high color saturation. As opposed to the security element of figure 7, it is shown here that, due to the change in the grating parameters, in particular the thickness of the highly refractive layer and the grating period, the tilt color in transmission can be selected.
Due to the fact that no color change occurs upon tilting perpendicularly to the an-gle of incidence, a security feature can be graphically designed such that a motif 15 is not visible under normal observation and appears only upon tilting. This can be done by arranging two grating regions 14,15 having the same grating profile in a manner in which they are rotated with respect to one another by 90 . This ar-rangement is shown in figures 9a and b.
The grating lines of the region 14, which forms the background, extend perpen-dicularly, while the grating lines in the region 15, forming the motif, extend hori-zontally. If the security element is tilted about the horizontal axis, the motif ap-pears. Further orientations of regions are also conceivable. Due to the finely incre-mentally oriented regions, it is also possible for example to form motion effects in transmission. To this end, reference is made by way of example to DE

-17-102011115589 Al. It is furthermore possible to form motifs using regions having different profiles, e.g. periods of the grating structure.
Furthermore, the metallic layer or the highly refractive layer can be formed not over the full area, but only in specific grating regions. Figures 10a and b show a motif of a butterfly and the number "12," wherein the square area around the number "12" contains no additional highly refractive coating (region 16 in figure 10b). Under normal observation, the motifs butterfly and the number "25"
cannot be seen, but the regions 16 and 17 appear in different colors. Upon tilting, the mo-tif additionally appears.
The security element can be used as a see-through window in banknotes. It can also be partially overprinted in color. One or both materials of the double layer can also be removed partially, e.g. by laser irradiation using ultrashort pulses. A
combination with highly refractive transparent holograms is also possible.
Such holograms can also act as reflection features. Part of the security element S
can be located on an absorbing substrate, with the result that said part serves only as a reflective feature and forms a contrast to the other part of the security element S
that is located in the region of the see-through window.
The security element can serve in particular as a see-through window in bank-notes or other documents. It can also be partially overprinted in color or the grat-ing regions can be partially demetalized or be designed without line gratings, such that such a region is completely metalized. Combinations with diffractive grating structures, such as holograms, are also conceivable.

- 18 -List of reference symbols 1 carrier or substrate 2 first line grating structure 3 first grating web 3a highly refractive material 3b metal 4 first grating gap 5 intermediate layer 6 second line grating structure 7 second grating web 7a highly refractive material 7b metal 8 second grating gap 10 cover layer 14-17 region = modulation depth or height t2, t3, t4 coating thickness a, b web and gap widths d period security element Li, L2 plane = incident radiation = reflected radiation T transmitted radiation = angle of incidence

Claims

Patent claims

1. A security element for producing valuable documents such as banknotes, checks or the like, the security element comprising:
- a dielectric substrate (1), - a first periodic line grating structure (2) that is embedded in the substrate (1) and made of a plurality of first grating webs (3), extending in a longitudinal di-rection and arranged in a first plane (L1), which have first grating gaps (4) located therebetween, and - a second line grating structure (6) of the same period (d) that is embedded in the substrate (1) and made of second grating webs (7), extending in the longitu-dinal direction, which have second grating gaps (8) located therebetween, wherein the second line grating structure (6) is located, relative to the first plane (L1), above the first line grating structure (2) in a parallel, second plane (L2) and wherein the second line grating structure (6) is formed to be inverted with respect to the first line grating structure (2) such that, in plan view of the first plane (L1), the second grating webs (7) are located above the first grating gaps (4) and the second grating gaps (8) are located above the first grating webs (3), characterized in that the security element (S) produces a color effect in transmission observation (T) and - the grating webs (3) of the first line grating structure (2) and the grating webs (7) of the second line grating structure (6) are formed each from a double layer, made of a layer of a highly refractive material (3a, 7a) and a layer of a me-tallic material (3b, 7b).

2. The security element as claimed in claim 1, wherein the highly refractive material (3a, 7a) has an index of refraction that is higher by at least 0.3 than that of the surrounding substrate (1).

3. The security element as claimed in one of the above claims, wherein the pe-riod (d) is from 200 to 700 nm.

4. The security element as claimed in one of the above claims, wherein the highly refractive material (3a, 7a) is selected from: Si, Ge, C, ZnS, ZnO, ZnSe, SiNx, SiO x, Cr2O3, Nb2O5, Ta2O5, Ti x O x and ZrO2, and wherein the metallic material (3b, 7b) is selected from: Al, Ag, Au, Cu, Cr and alloys thereof.

5. The security element as claimed in one of the above claims, wherein the spacing (h) between the planes (L1, L2) is from 100 nm to 500 nm.

6. The security element as claimed in one of the above claims, wherein the se-curity element (S) has, in top view onto the plane (L1), at least two regions (14, 15), the periods (d) of which differ.

7. The security element as claimed in one of the above claims, wherein the se-curity element has, in top view onto the plane (L1), at least two regions (14, 15), of which the directions of the grating webs (3, 7) differ, preferably by 90 degrees.

8. The security element as claimed in one of the above claims, which is in the form of a transmissive element, in particular as a window element for a valuable document.

9. A valuable document having a security element (S) as claimed in one of the above claims, wherein the valuable document has a window or a region that is in-tended for observation in transmission, which window or which region is covered by the security element (S).