Optically variable security element with 5-layer stack, data carrier and production method

Abstract

The invention relates to an optically variable security element (20), in particular security film or security thread, with a 5-layer stack (21) comprising, in this order, a first absorber layer (23), a first dielectric spacer layer (24), a second absorber layer (25), a second dielectric spacer layer (26) and a reflection layer (27). The invention here provides for the first and the second dielectric spacer layers (24, 26) to have different layer thicknesses in order to generate a specific IR reflection signature of the security element.

Description

[0001] Optically variable security element with 5-layer stack, data carrier and manufacturing process

[0002] The invention relates to an optically variable security element with a 5-layer stack comprising, in this order, a first absorber layer, a first dielectric spacer layer, a second absorber layer, a second dielectric spacer layer, and a reflection layer. The invention also relates to a data carrier with such a security element and a method for producing such a security element.

[0003] Data storage media, such as valuables or identification documents, or other valuables, such as branded goods and packaging or outer packaging of high-quality branded goods, are often provided with security elements for security purposes. These elements allow the authenticity of the data storage media to be verified and also serve as protection against unauthorized reproduction.

[0004] Security elements with viewing-angle-dependent effects play a special role in authenticity assurance, as these cannot be reproduced even with the most modern copying machines. Color-shifting security elements are often used, which feature a stack of thin, well-controlled layers to produce angle-dependent interference colors or color changes.

[0005] The security function of optically variable security elements can be increased if they are additionally designed to be machine-readable. When used on banknotes, the security elements can then be checked both visually by a human user and sensorically on slow deposit devices and on fast banknote processing machines. On banknote processing machines, additional testing can improve both the detection of counterfeit banknotes and the condition assessment of genuine banknotes. The color-shifting security elements usually consist of a three-layer resonator consisting of a chromium absorber layer, a dielectric spacer layer made of SiCb, and an aluminum reflector layer. The layer stacks exhibit reflected interference maxima not only in the visually visible spectral range but also in the near infrared, which can be used for machine authentication.The interference maxima in the visible and near infrared range are intrinsically coupled to each other via the thickness of the SiCb spacer layer.

[0006] From EP 0472371 A1, a double resonator with a 5-layer stack is also known, in which two periods of absorber layer and dielectric spacer layer are arranged on a reflector layer in order to suppress peaks in the reflection spectrum.

[0007] Based on this, the object of the invention is to provide a security element of the type mentioned at the outset with increased security against counterfeiting.

[0008] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0009] According to the invention, in a generic security element, the first and second dielectric spacer layers have different layer thicknesses in order to generate a specific IR reflection signature of the security element.

[0010] Compared to a conventional 3-layer stack, the use of a 5-layer dual resonator offers additional design freedom, allowing both to maintain a desired or proven color-shift effect in the visible spectral range and to generate a specific infrared reflection signature in the near infrared range that is modified compared to conventional 3-layer structures. This specific signature is provided in particular by the different thicknesses of the two dielectric spacer layers. Preferably, the different thicknesses of the two dielectric spacer layers differ by more than 10% of the thickness of the thinner of the two dielectric spacer layers.

[0011] The use of different thicknesses of the two dielectric spacer layers in the 5-layer structure results in the reflection spectrum in the near infrared differing particularly strongly from the reflection spectrum of a 3-layer structure with a comparable color shift effect in the visible spectral range.

[0012] A security element according to the invention can then exhibit the same functionality in the visible spectral range as a conventional security element with a three-layer structure, but can be distinguished from a conventional security element during machine authentication due to its specific IR reflection signature. By varying the distribution between the two dielectric spacer layers, several different specific IR reflection signatures can be generated with the same total dielectric thickness. With a clever choice of the thickness ratio and the manufacturing process, the security elements according to the invention can also exhibit better properties, in particular higher color purity and more brilliant colors, than security elements with conventional three-layer structures and a comparable color-shift effect.

[0013] In an advantageous embodiment, the first and second dielectric spacer layers have a thickness ratio in the ratio of small whole numbers, such as approximately 2:1 (or 1:2) or approximately 3:1 (or 1:3) or approximately 4:1 (or 1:4), since this achieves particularly pronounced modulations and thus a particularly characteristic spectral profile in the near infrared, while achieving brilliant color effects in the visible spectral range. The inventors' experiments have shown that a thickness ratio of approximately 2:1 or 1:2 is particularly advantageous for this purpose. With such a ratio, the most attractive spectra in the visible spectral range and the most striking differences in the near infrared could be achieved.

[0014] The thicker of the two dielectric spacer layers is preferably in a thickness range between 200 nm and 600 nm, as interference colors with the greatest saturation can be achieved in this thickness range. With a layer thickness of less than 200 nm in the thicker layer, the reflection maxima are already very close to the edge of the visible spectral range; with a layer thickness of more than 600 nm in the thicker layer, numerous maxima arise in the visible spectral range, which also lead to reduced saturation. Thicker layers are also disadvantageous in terms of producibility and cost-effectiveness.

[0015] Furthermore, it has been found advantageous if the dielectric spacer layer located closer to the reflective layer is the thinner of the two spacer layers. If the 5-layer stack is applied to a substrate, the thinner dielectric spacer layer is advantageously the dielectric spacer layer far from the substrate, and the thicker dielectric spacer layer is the dielectric spacer layer closest to the substrate. With such a sequence of thick and thin layers, the inventors found particularly pronounced extreme values ​​in the reflection spectra.

[0016] In an advantageous embodiment, the first and / or second dielectric spacer layer is formed from an inorganic material, preferably from one of the materials SiCl, Al3O2, MgF2, CaF2, and SiON. In a particularly preferred embodiment, both dielectric spacer layers are formed from SiCl2.

[0017] In further advantageous embodiments, at least one of the dielectric spacer layers is advantageously formed from a polymer-based material, preferably from a material based on nitrocellulose, epoxy, polyester, rosin, acrylate, alkyd, melamine, PVA, PVC, isocyanate, or urethane systems. In a particularly preferred embodiment, one, for example the first, dielectric spacer layer is formed from SiCb, and one, for example the second, dielectric spacer layer is formed from a polymer-based material.

[0018] The reflective layer is preferably made of aluminum or a metal with a non-color appearance and high reflectivity in the visible range. For example, the reflective layer can be made of one of the metals Ti, Ta, Zn, Mo, W, Ni, Ir, or Pd.

[0019] In an advantageous embodiment, the five-layer stack is arranged on a carrier film, preferably in such an orientation that the first absorber layer represents the layer of the layer stack closest to the carrier film. The carrier film can be formed, for example, from a PET film and is advantageously transparent, allowing viewing through the carrier film.

[0020] The carrier film can be unstructured or structured, whereby the 5-layer stack can also be applied, in particular, to the structured side of the carrier film. The carrier film is advantageously provided with micro-optical structures, in particular with embossed micro-optical structures, preferably with a micromirror array and / or nano-optical structures, such as holograms. In addition to the color-shift effect, the micro-optical structures can generate further effects, for example, motion effects, motif-changing effects, 3D effects, or the like. In their experiments, the inventors discovered that although the micro-optical structures generate microscopically locally modified viewing angles, they hardly interfere with the machine readability of the security element in the near infrared.

[0021] The invention also includes a data carrier with an optically variable security element of the type described above. The data carrier can in particular be a value document, such as a banknote, in particular a paper banknote, a polymer banknote or a composite film banknote, a share, a bond, a certificate, a voucher, a cheque, a seal, a tax stamp, a high-value admission ticket, but also an identification card, such as a credit card, a bank card, a cash payment card, an authorization card, an identity card or a passport personalisation page.

[0022] The invention further includes a method for producing an optically variable security element of the type described, in which a 5-layer stack with the layer sequence of first absorber layer, first dielectric spacer layer, second absorber layer, second dielectric spacer layer, and reflection layer is applied to a substrate, and in which the first and second dielectric spacer layers are applied in different layer thicknesses to generate a specific IR reflection signature of the security element. The substrate can, in particular, be the aforementioned carrier film.

[0023] In an advantageous process, the first absorber layer, the first dielectric spacer layer, the second absorber layer and the second dielectric spacer layer are applied in an inline system without leaving the vacuum.

[0024] Advantageously, the first and second absorber layers are applied by electron beam evaporation or sputter deposition, in particular by DC magnetron sputtering.

[0025] Also advantageously, the first and / or second dielectric spacer layer is applied by electron beam evaporation or RF sputtering or reactive pulsed DC magnetron sputtering.

[0026] In an advantageous development, the second dielectric spacer layer is a polymer-based dielectric layer applied by printing or by slot-die coating in a continuous process. Further embodiments and advantages of the invention are explained below with reference to the figures, which are not drawn to scale or proportions for the sake of clarity.

[0027] They show:

[0028] Fig. 1 is a schematic representation of a banknote with two security elements according to embodiments of the invention,

[0029] Fig. 2 schematically shows the structure of a security element according to an embodiment of the invention,

[0030] Fig. 3 the structure of a conventional security element with 3-layer resonator,

[0031] Fig. 4 shows the reflection spectrum of the conventional 3-layer structure of Fig. 3 in comparison with the reflection spectrum of the inventive 5-layer structure according to Fig. 2,

[0032] Fig. 5 shows the reflection spectra of two security elements according to the invention with a 5-layer stack, each according to the principle of Fig. 2,

[0033] Fig. 6 shows a comparison of the reflection spectra of a conventional 3-layer structure with a 5-layer structure according to the invention, each under perpendicular and oblique illumination, and Fig. 7 shows a comparison of a reflection spectrum of a color-shift layer stack on an unstructured PET superstrate and a reflection spectrum of the same layer stack on a PET superstrate provided with embossed micromirrors.

[0034] The invention will now be explained using the example of security elements for banknotes. Figure 1 shows a schematic representation of a banknote 10 provided with two security elements 12 and 14 according to embodiments of the invention. The first security element represents a security thread 12, which can be applied to the surface of the banknote or partially embedded in the banknote substrate.

[0035] The second security element is formed by an adhesive-applied transfer element 14 of any desired shape. The security element 14 can also be in the form of a cover foil arranged over a window area or a through-opening of the banknote. However, it is understood that the invention is not limited to transfer elements and security threads on banknotes, but can be used for all types of security elements, for example, labels on goods and packaging or for securing documents, ID cards, passports, credit cards, health cards, and the like.

[0036] The structure and appearance of security elements according to the invention in comparison to conventional security elements will now be explained in more detail with reference to Figures 2 to 7.

[0037] Figure 2 schematically shows the structure of a security element 20 according to an embodiment of the invention. In the security element 20, a 5-layer stack 21 is applied to a 12 μm thick PET carrier film 22, which, starting from the carrier film 22, comprises in this order a 3 nm thick first absorber layer 23 made of chromium, a 140 nm thick first dielectric spacer layer 24 made of SiO?, a 5 nm thick second absorber layer 25 made of chromium, a 280 nm thick second dielectric spacer layer

[0038] 26 made of SiCb, and an opaque, 50 nm thick reflection layer 27 made of aluminum.

[0039] Figure 4 shows a comparison of the reflection spectra of the 5-layer structure according to the invention in Fig. 2 with a conventional 3-layer structure according to Fig. 3. In the conventional security element 30, a 3-layer stack 31 is applied to a 12 gm thick PET carrier film 32, which, starting from the carrier film 32, comprises in this order a 5 nm thick absorber layer 33 made of chromium, a 400 nm thick dielectric spacer layer 34 made of SiCb and an opaque, 50 nm thick reflection layer 37 made of aluminum.

[0040] In the 5-layer stack 21, the layer thicknesses are selected such that the sum of the layer thicknesses of the two dielectric spacer layers 24, 26 is equal to the layer thickness of the individual dielectric spacer layer 34 of the 3-layer structure. This ensures that the two different layer stacks have a reflection spectrum in the visible spectral range that is as comparable as possible. The layer thicknesses of the PET carrier films 22, 32 and the opaque reflection layers 27, 37 are each the same.

[0041] The reflection spectra of the two structures were calculated using the IMD software (David L. Windt, "IMD — Software for modeling the optical properties of multilayer films", Computers in Physics 12, 360 (1998)). Using optical constants, particularly refractive and extinction indices from the literature, the various materials were modeled as individual layers in a layer stack, and the resulting reflection and transmission spectra were calculated using the Fresnel equations. The angles of incidence, layer thicknesses, and layer boundary transitions are also included in the calculation. Figure 4 shows, in diagram 40, the reflection spectrum 42 of the conventional 3-layer structure of Fig. 3 with solid lines compared to the reflection spectrum 44 of the inventive 5-layer structure according to Fig. 2 with dashed lines. Both reflection spectra were calculated for perpendicular light incidence 28 (see Fig. 2).Diagram 40 shows the reflectivity as a function of wavelength in the visible spectral range from 380 nm to 780 nm and in the near infrared range from 780 nm to 2000 nm.

[0042] In the visible spectral range, the two reflection spectra 42, 44 exhibit a common, dominant reflection peak 46 at approximately 460 nm. However, the reflection spectra differ fundamentally from each other in the near infrared at wavelengths above 700 nm. While the conventional 3-layer structure exhibits another reflection maximum 48 at approximately 930 nm in the near infrared, this is greatly suppressed in the 5-layer structure according to the invention, and the reflection even exhibits a minimum 50 there, while a broad secondary maximum 52 develops at approximately 1200 nm.

[0043] While offering largely identical functionality in the visible spectral range, the inventive 5-layer structure exhibits a significantly different, characteristic spectral profile in the near infrared. This different spectral profile can be verified with reflection or remission sensors, such as those typically used on banknote processing machines, and used as an additional authentication feature.

[0044] In addition to the human characteristic that can be tested without aids, namely the color shift effect in the visible spectral range, the security element 20 thus also provides a characteristic machine-readable authenticity feature for testing in the infrared spectral range.

[0045] Authenticity testing with remission or reflection sensors can be carried out with simple sensors with only a few, for example two, test wavelengths, as used in deposit machines, for example, as well as with more complex sensors with higher spectral resolution, as used in particular in larger processing machines.

[0046] To achieve the most pronounced extreme values ​​in the reflection spectra and thus brilliant colors, it is advantageous to coordinate the layer thicknesses of the two dielectric spacer layers 23, 25. A particularly appealing appearance is achieved when the thickness ratio of the dielectric spacer layers is approximately 2:1, as in the exemplary embodiment in Fig. 2, since constructive and destructive interference in the visible spectral range is then maximized, resulting in a visually impressive, brilliant color impression.

[0047] Furthermore, thickness ratios of the dielectric spacer layers in the ratio of small integers such as approximately 2:1 (or 1:2) or approximately 3:1 (or 1:3) or approximately 4:1 (or 1:4) are particularly advantageous, since they achieve particularly pronounced modulations and thus a particularly characteristic spectral profile in the near infrared, while at the same time providing brilliant color effects in the visible spectral range.

[0048] Particularly pronounced extreme values ​​in the reflection spectra arise when the dielectric spacer layer near the substrate is the thicker of the two spacer layers. Figure 5 shows, for illustration, diagram 60, the reflection spectra 62, 64 of two security elements according to the invention with a 5-layer stack according to the principle of Figure 2. In both variants, the thickness ratio of the two dielectric spacer layers is 2:1 and the total layer thickness is 600 nm. The structures differ only in the sequence of the thicker and thinner dielectric spacer layers.

[0049] The solid reflection spectrum 62 represents the simulated reflection spectrum of a first variant of the 5-layer stack, in which the dielectric spacer layer 24 near the substrate is the thicker spacer layer and is formed by a 400 nm thick SiC>2 layer, while the dielectric spacer layer 26 far from the substrate is thinner and is formed by a 200 nm thick SiCh layer.

[0050] The dashed reflection spectrum 64 represents the simulated reflection spectrum of a second variant of the 5-layer stack, in which the dielectric spacer layer 24 near the substrate is the thinner spacer layer and is formed by a 200 nm thick SiO2 layer, while the dielectric spacer layer 26 far from the substrate is thicker and is formed by a 400 nm thick SiO2 layer. The carrier film 22, the absorber layers 23, 25, and the reflection layer 27 are the same in both variants and are designed as in Fig. 2.

[0051] As can be seen from the reflection spectra 62, 64, the first variant, in which the dielectric spacer layer near the substrate is the thicker layer, leads to significantly more pronounced minima in both the visible and near-infrared spectral ranges. The first variant therefore results in more brilliant colors in the visible and allows for easier discrimination in the infrared, and is therefore particularly preferred.

[0052] With an advantageous production of the 5-layer stacks according to the invention, in addition to the modified IR signature, narrower half-widths of the interference maxima can also be obtained.

[0053] In principle, the 5-layer stacks can be manufactured in conventional systems used for the production of security elements with a 3-layer structure. This simply involves alternating the deposition of the absorber layer, for example a chromium layer, and the deposition of the dielectric spacer layer, for example the SiCb layer, to create a double resonator. However, it has proven particularly advantageous to carry out the manufacturing process in an inline system, in which the dielectric spacer layer is deposited together with the absorber layer in a single pass, in particular without leaving the vacuum. In the described 5-layer structures, the first absorber layer 23 made of chromium is first deposited sequentially onto the PET substrate 22 by electron beam evaporation or sputter deposition, in particular DC magnetron sputtering from the tube target.In the next deposition chamber, the first dielectric spacer layer 24 made of SiCh is deposited, preferably by electron beam evaporation, followed by another deposition chamber for the second absorber layer 25 made of chromium, and another SiC>2 evaporation chamber for the second dielectric spacer layer 26. Finally, the aluminum reflection layer 27 is evaporated in the usual manner. The inline production of the second absorber layer and the second dielectric spacer layer is also particularly advantageous from an economic perspective.

[0054] Diagram 70 of Fig. 6 shows a comparison of the reflection spectra of a conventional 3-layer structure (black curves 72, 74) with a 5-layer structure according to the invention (gray curves 76, 78). The solid lines 72, 76 each show the reflection spectrum for perpendicular light incidence, while the dashed lines 74, 78 show the reflection spectrum for oblique light incidence at 45°.

[0055] The layer thicknesses of the chromium layers and the SiCb layers were 5 nm Cr and 300 nm SiC>2 for the 3-layer structure and 2 nm Cr, 300 nm SiC>2, 5 nm Cr and 150 nm SiO2 for the 5-layer structure.

[0056] As can be seen in Fig. 6, the reflection lines in the inventive 5-layer stack (gray curves 76, 78) are very similar to the reflection lines of the 3-layer stack, resulting in visually comparable color effects in the visible spectral range. However, the IR signature of the 5-layer structure differs dramatically from the signature of conventional designs with a 3-layer structure due to the suppression of the NIR peak at 900-1000 nm.

[0057] In a further advantageous manufacturing variant, a conventional, inorganic, first dielectric spacer layer 24, such as a vapor-deposited SiCb layer, is combined with a second, thicker, polymer-based dielectric spacer layer 26. The latter can be produced, for example, by printing, such as by gravure printing, or by slot-die coating in a continuous process.

[0058] This manufacturing variant offers reduced process effort and reduced equipment costs compared to multiple vacuum coatings. At the same time, brilliant color changes combined with unconventional IR reflection spectra can be achieved. For the polymer-based dielectric spacer layer, a coating based on nitrocellulose, epoxy, polyester, rosin, acrylate, alkyd, melamine, PVA, PVC, isocyanate, or urethane systems is possible.

[0059] As described above, the 5-layer stacks according to the invention can be combined, for example, with embossed micro-optical structures to create motion effects, motif changes, 3D effects, or the like. Although the micro-optical structures result in a locally modified viewing angle, they hardly interfere with the machine readability of the security element in the near infrared.

[0060] Figure 7 shows, in diagram 80, a comparison of a measured reflection spectrum 82 of a color-shift layer stack on an unstructured PET superstrate and a measured reflection spectrum 84 of the same layer stack on a PET superstrate provided with embossed micromirrors. As can be seen from Fig. 7, the optical micromirror structures have only a quantitative influence on the observed reflection spectrum, while qualitatively the essential structures of the spectral characteristics are retained. Although the spectral positions of the maxima and minima are slightly shifted towards shorter wavelengths, this is due to the shortened effective optical path length due to the micromirrors. This shift can be compensated for by targeted adaptation of the layer stack used, so that a fixed color impression or a fixed infrared signature can be set regardless of the specific design of the micromirrors.The already rather disturbing superstrate interference at longer infrared wavelengths is even suppressed by the locally different reflection of the micromirrors.

[0061] List of reference symbols

[0062] 10 banknotes

[0063] 12 security threads

[0064] 14 Transfer element

[0065] 20 security element

[0066] 21 5-layer stacks

[0067] 22 PET carrier film

[0068] 23 first absorber layer

[0069] 24 first dielectric spacer layer

[0070] 25 second absorber layer

[0071] 26 second dielectric spacer layer

[0072] 27 Reflective layer

[0073] 30 conventional security element

[0074] 31 3-layer stacks

[0075] 32 PET carrier film

[0076] 33 Abs orber layer

[0077] 34 dielectric spacer layer

[0078] 37 reflective layer

[0079] 40 Diagram

[0080] 42 Reflection spectrum 3-layer stack

[0081] 44 Reflection spectrum 5-layer stack

[0082] 46 reflection peak

[0083] 48 further reflection maximum

[0084] 50 minimum

[0085] 52 secondary maximum

[0086] 60 diagram

[0087] 62 Reflection spectrum first variant

[0088] 64 Reflection spectrum second variant

[0089] 70 Diagram 72 Reflection spectrum 3-layer stack, vertical

[0090] 74 Reflectance spectrum 3-layer stack, oblique

[0091] 76 Reflectance spectrum 5-layer stack, vertical

[0092] 78 Reflectance spectrum 5-layer stack, oblique 80 Diagram

[0093] 82 Reflection spectrum on unstructured superstrate

[0094] 84 Reflection spectrum on structured superstrate

Claims

Patent claims 1. Optically variable security element (20), in particular security film or security thread, with a 5-layer stack (21) comprising, in this order, a first absorber layer (23), a first dielectric spacer layer (24), a second absorber layer (25), a second dielectric spacer layer (26) and a reflection layer (27), characterized in that the first and second dielectric spacer layers (24, 26) have different layer thicknesses in order to generate a specific IR reflection signature of the security element.

2. Security element (20) according to claim 1, characterized in that the first and second dielectric spacer layers (24, 26) have a thickness ratio of 4:1 or 1:4 or 3:1 or 1:3 or 2:1 or 1:2, preferably 2:1 or 1:

2.

3. Security element (20) according to claim 1 or 2, characterized in that the thicker of the two dielectric spacer layers lies in a thickness range between 200 nm and 600 nm.

4. Security element (20) according to at least one of claims 1 to 3, characterized in that the first and / or second dielectric spacer layer (24, 26) is formed from an inorganic material, preferably from one of the materials SiCh, Al3O2, MgF2, CaF2 and SiON, particularly preferably that both dielectric spacer layers are formed from SiO2.

5. Security element (20) according to at least one of claims 1 to 4, characterized in that at least one of the dielectric spacer layers is formed from a polymer-based material, preferably from a material based on nitrocellulose, epoxy, polyester, rosin, acrylate, alkyd, melamine, PVA, PVC, isocyanate or urethane systems, particularly preferably, a dielectric spacer layer made of SiO? and a dielectric spacer layer made of a polymer-based material.

6. Security element (20) according to at least one of claims 1 to 5, characterized in that the reflection layer (27) is formed from aluminum or one of the metals Ti, Ta, Zn, Mo, W, Ni, Ir, Pd.

7. Security element (20) according to at least one of claims 1 to 6, characterized in that the 5-layer stack (21) is arranged on a carrier film (22), preferably in such an orientation that the first absorber layer (23) represents the layer of the layer stack closest to the carrier film (22).

8. Security element (20) according to claim 7, characterized in that the carrier film (22) is provided with micro-optical structures, in particular with embossed micro-optical structures, preferably with a micro-mirror array and / or nano-optical structures.

9. Data carrier (10) with an optically variable security element (20) according to at least one of claims 1 to 8.

10. A method for producing an optically variable security element (20) according to one of claims 1 to 8, in which a 5-layer stack (21) with the layer sequence first absorber layer (23), first dielectric spacer layer (24), second absorber layer (25), second dielectric spacer layer (26) and reflection layer (27) is applied to a substrate (22), characterized in that the first and second dielectric spacer layers (24, 26) are applied in different layer thicknesses in order to generate a specific IR reflection signature of the security element.

11. The method according to claim 10, characterized in that the first absorber layer (23), the first dielectric spacer layer (24), the second absorber layer (25) and the second dielectric spacer layer (26) are applied in an inline system without leaving the vacuum.

12. The method according to claim 10 or 11, characterized in that the first and second absorber layers (23, 25) are applied by electron beam evaporation or sputter deposition, in particular by DC magnetron sputtering.

13. The method according to at least one of claims 10 to 12, characterized in that the first and / or second dielectric spacer layers (24, 26) are applied by electron beam evaporation.

14. The method according to at least one of claims 10 to 13, characterized in that the second dielectric spacer layer is a polymer-based dielectric layer which is applied by printing or by slot die coating in continuous operation.

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