HK1190196B - Two-dimensionally periodic, colour-filtering grating - Google Patents

Description

Two-dimensional periodic color filtering grating

The invention relates to a two-dimensional periodic color filter grating. One-dimensional periodic gratings as security or security elements for value documents are known, for example, from DE102009012299a1, DE102009012300a1 or DE102009056933a 1. When the grating profiles are designed to exhibit resonant effects in the visible wavelength range, they may have color filtering properties in the sub-wavelength range. Such color filtering properties are known not only for reflective sub-wavelength structures but also for transmissive sub-wavelength structures. These structures have a strong polarizing effect on the reflection or transmission of the incoming light beam. When the incident light is unpolarized, the color saturation of the grating is significantly reduced. In addition, the color has a relatively large relationship with angle upon reflection or transmission of such a subwavelength grating.

Two-dimensional periodic pore structures are described in some scientific literature that have filtering properties for incident light in a sub-wavelength range. For this purpose, reference is made to the following original documents: T.W.Ebbesen, H.J.Lezec, H.F.Ghaemi, T.thio, and P.A.wolf,, expression vertical optical transmission through wall-wall long holes, Nature,667-, phys. Rev. B75, 24421 (2007). The Hole-Arrays consists of opaque metal films. DE102007016394a1 proposes the use of such a structure as a security or security element in documents of value for the purpose of verifying authenticity.

Two-dimensional periodic gratings are also known which allow color filtering in the case of high color saturation in a comparatively large viewing angle range, if the ground color is red, green or blue. The published documents b. -h.cherong, o.n.prudnikov, e. -h.cho, h. -s.kim, Jaeho Yu, Young-Sang Cho, Hwan-Young choice, and Sung Tae Shin, "High angular prism using subwavelengthgraphing", appl.phys.lett.94,213104(2009) describe gratings with cubic protuberances which have a pronounced bandpass characteristic. The protuberances are composed of amorphous silicon and are disposed on a glass substrate. The method of replication of such gratings is shown in the publications E. -H.Cho, H. -S.Kim, Byong-Ho Cheng, Prudnikov Oleg, Wenxu Xianyua, Jin-Seung Sohn, Dong-Joon Ma, Hwan-YoungChoi, No-Chemol Park, and Yong-Pil Park, "Two-dimensional photo crystal filter translation", Opt.Expresso 17,8621-8629(2009), and E. -H.Cho, H. -S.Kim, Jin-Seung Sohn, Chang-Youl Moton, No-Chemol Park, and Yong-PilPark 722, "Nanorimed photo crystal filter for use", Op.27712, Op.Expresso 2718. The publications Yan Ye, Yun Zhou, and Linsen Chen, "Color filter based on a two-dimensional radiometer metrology", appl. Opt.48,5035-5039(2009) and Yan Ye, Heng Zhang, Yun Zhou, and Linsen Chen, "Color filter based on a sub-micrometer captured rating", Opt. Commun.,283,613-616(2010) propose a Color-filtered two-dimensional periodic grating in which cubic-shaped protrusions composed of aluminum and zinc sulfide or aluminum are on a high refractive layer. No manufacturing methods available for such structures have been known to date. A grating with the same surface geometry is described in WO2010/126493a 1. However, it does not exhibit significant color filtering characteristics. Finally, two-dimensional periodic gratings are known from EP1434695B1, which have light-absorbing properties. This structure is based on a continuous metal film and therefore has no appreciable light transmission. Also known for no colour-filters.

Although two-dimensional periodic sub-wavelength gratings with non-coherent surfaces are known to exhibit significant color filtering properties at the same time as large angular tolerances, they are very cumbersome to manufacture. It is not possible to use a simple replication method. This construction is therefore unsuitable as a security element for value documents, in particular banknotes, since a large number of simple production steps are required.

It is therefore an object of the present invention to provide a two-dimensional color filter grating which, on the one hand, has good color filter properties and, on the other hand, can be produced by an economical replication method.

This object is achieved according to the invention by a two-dimensional periodic color filter grating having a coherent, in particular metallic, base layer which defines a grating plane and on which a two-dimensional regular pattern of individual, in particular metallic, surface elements of high refractive index extends parallel to the grating plane and is spaced apart from the base layer by an intermediate dielectric, which is greater than the thickness of the base layer and the surface elements, wherein the regular pattern has a period in at least two directions which extend parallel to the grating plane of between 100nm and 800nm, preferably between 200nm and 500 nm.

The grating is provided with a highly refractive substrate and high refractive index surface elements arranged on top of the substrate. The high refractive properties of the substrate or surface element are achieved by selecting the appropriate material. In addition to metallic materials, the use of silicon, zinc sulfide or titanium dioxide, in particular, is also conceivable in this connection. Within this specification, the term "metallic" is understood to be synonymous with "highly refractive", unless expressly stated otherwise.

What is important for the effect of the grating is that the discontinuous surface elements are arranged in a pattern on the continuous metal layer. When a two-dimensional periodic grating having a profile with vertical sides is vertically metal-evaporated, an unclosed metal film is formed on a flat or mesa surface on the upper side of the grating. A continuous metal film is formed on the lower grating surface (bottom surface). The elevations of the profile are metallized only on the plateau here.

The non-evaporated grating structure here consists of a dielectric metal, which has a refractive index of, for example, approximately 1.5. In this case, in particular, plastic films are suitable as substrates, for example PET films. The actual basic structure is, for example, also formed in plastic, preferably in a UV lacquer (UV-curable lacquer or light-curable coating). After evaporation the structure was finally filled with UV paint and covered with a mask. A layered structure is thus present, in which the upper side and the lower side have substantially the same refractive index.

Furthermore, the coating is not limited to a single metal layer, but also a plurality of layers, in particular three layers, can be considered. It is known that multilayer coated one-dimensional periodic gratings can strongly filter color not only in reflection but also in transmission by constituting Fabry-Perot resonators. In the case of three layers the following layering is particularly preferred: two semi-transparent metal layers with a dielectric spacer layer between them, or two high refractive layers with a low refractive layer between them. The following materials are conceivable for the metal layer: al, Ag, Pt, Pd, Au, Cu and alloys thereof. E.g. ZnS, ZnO, TiO₂，ZnSe，SiO，Ta₂O₅Or silicon is suitable as the high refractive layer. Providing SiO as a low refractive layer₂，Al₂O₃Or MgF₂。

In a particularly rational production method, a dielectric as a basic structure is first appropriately structured and then coated. The bottom layer has a hole in the area under each of the face elements. This is also advantageous because the optical effect is then formed even in transmission.

Preferably, the grating is embedded in an embedding dielectric, which preferably has the same refractive index as the intermediate dielectric that constitutes the basic structure and separates the bottom layer from the surface elements. The refractive index may be, for example, between 1.4 and 1.6.

It has been demonstrated that the color effect of a two-dimensional color filter grating is related to the periodicity of the pattern. This may be sufficient for generating a colored symbol or image. For this purpose, the surface filling factor and/or the distance between the surface element and the underlayer can be varied locally. In particular, a surface element group formed from a plurality of surface elements of the same size can be designed such that the desired color effect occurs. This group then constitutes a sub-pixel. A plurality of sub-pixels are provided with different color characteristics by corresponding geometric designs and then combined into one pixel. This allows to represent a color image. The different colors can be changed by correspondingly locally changing one or more parameters of the grating (the distance of the surface elements in the two spatial directions and the distance of the surface elements from the substrate).

A true color image can be made by pixel-wise color mixing of primary colors, such as RGB (red, green and blue) colors, within the sub-pixel regions. The advantage of such a structure compared to conventional printing techniques is that very fine structuring up to the micrometer range can be carried out here. This fine structuring is particularly suitable for use in a moir é magnifier, for example by designing the grating such that it provides a microscopic image for the moir magnifier. The large angular tolerance of the two-dimensional periodic grating described above plays a very advantageous role in microlens assemblies, since microlenses in moire magnification devices have a small focal length while the aperture is relatively large. Thus, the structure described herein in combination with the micro-lenses exhibits greater color saturation than heretofore known one-dimensional periodic sub-wavelength structures.

The two-dimensional periodic grating can be used in particular in security elements (i.e. security elements) of value documents. It may be integrated in a safety thread, tear thread, safety belt, safety strip, stripe or label, among others. In particular, the security element provided with a grating may span a transparent area or cut-out.

The light barrier as a security element can in particular be part of a precursor of a value document which is not yet suitable for circulation, but the value document can also have further security elements. A value document is understood to mean, on the one hand, a document with a two-dimensional raster. On the other hand, the value documents can also be other documents or articles provided with a two-dimensional raster, so that these value documents have an irreproducible authenticity feature and can be checked for authenticity and protected against undesired copying. Other examples of value documents are chip cards or security cards, such as bank cards or credit cards or certificates.

The features mentioned above and those yet to be explained below can of course be used not only in the combination indicated, but also in other combinations or alone without departing from the scope of the present invention.

The invention is explained in more detail below by way of example with the aid of the accompanying drawings, which also disclose important features of the invention. Wherein:

fig. 1 shows a schematic perspective view of a first embodiment of a color filter grating;

FIG. 2 shows a further development of the grating shown in FIG. 1;

FIG. 3 shows a variation of the grating of FIG. 2;

figures 4-5 schematically illustrate the operation of a colour filter grating;

FIG. 6 shows a further development of the grating shown in FIG. 3;

FIGS. 7-11 show graphs relating to the filtering characteristics of different color filter gratings;

FIG. 12 schematically illustrates a color filter grating for displaying an image;

fig. 13-15 show graphs of the filtering characteristics of different color filter gratings.

Fig. 1 shows a color filter grating 1 which can be used, for example, as a security or security element in value documents. In order to produce the color filter grating 1, the carrier 2 is provided with a profile having vertical sides. The carrier 2 is meant to be a basic structure. The profile is designed to form a pattern 6 of the columns 4 on the upper side of the carrier 2. The carrier is composed of a dielectric and is coated with a metal layer 3, which is applied as a base layer 3 on the surface of the carrier 2 and as a coating 5 on the pillars 4. Since the sides are vertical, the sides are not plated.

In the pattern 6, the pillars 4, which are designed here by way of example only as cuboidal and in particular can be cylindrical (not necessarily cylindrical) elevations, are arranged in the form of a two-dimensional periodic grating in which a period p is present in two mutually perpendicular directions in a grating plane defined by the grating via the underlayer 3₁And p₂. Dimension of the pillar 4 or protuberance in the bottom surface by s₁And s₂And marking. The base layer 3 and the coating layer 5 have a layer thickness t. The plating 5 arranged in a pattern 6 is spaced apart from the upper side of the bottom layer 3 by a distance h-t through the height h of the pillars 4. The height h of the pillars of the shaped support 2 is here greater than the layer thickness t, so that the metal layer is interrupted and the coating 5 is discontinuous. This results in a metal structure consisting of a base layer 3, which defines a grating plane above which the coating 5 is located. The distance between the coating 5 and the base layer 3 is produced here by the dielectric pillars 4.

Period p₁And p₂In the wavelet range, i.e. in the range between 100nm and 800nm, preferably between 200nm and 450nm or 600 nm. Fill factor s₁/p₁And s₂/p₂Between 0.2 and 0.8, preferably between 0.3 and 0.7. In order to achieve polarization-independent color filtering, the profile parameters of the two spatial directions are selected to be as uniform as possible, i.e. p₁=p₂And s₁=s₂. Although this is not required. Also, the periodic directions are perpendicular to each other in the illustrated embodiment. This is also optional. Spatially asymmetric arrangements of profiles and periodicities are also contemplated. In other words, the pattern 6 need not be a cartesian pattern as represented in fig. 1. The upright 4 can also be designed asymmetrically.

Fig. 2 shows a further development in which the pattern 6 is embedded in an embedding dielectric 7. This has a significant advantage in use, since the surface of the grating 1 is now smooth.

Fig. 3 shows a grating 1 whose posts 4 are cylindrical in design. This shape is particularly suitable for color filtering of unpolarized light as in the construction of fig. 1 or 2. Variations other than the cube shape of fig. 1 or the circular shape of fig. 3 are equally possible, for example variations formed by corner rounding.

Fig. 3 and 4 show the operation of the grating 1 in the example of the construction shown in fig. 2. In which figure 4 shows the case when the beam E is incident on the upper side 9 of the grating. In fig. 5, the light is shown entering from the lower side 10. The grating 1 reflects the incoming beam E to form a reflected beam R and emits partly as a transmitted beam T. The main difference between illumination from the top side 9 and from the bottom side 10 is that the incident beam E first hits the periodic coating 5 arranged in a pattern 6 from the top side 9. Conversely, from the underside 10, the pattern of the holes 8 in the bottom layer 3 is directly illuminated. This distinction has obvious consequences with regard to the reflection behavior, in particular with regard to the color impression.

By applying a metal layer to the structured carrier 2, the base layer 3 is provided with holes 8 below the coating 5, i.e. in the region of the pillars 4. The grating 1 thus has a Hole-Array in the metal layer 3, where the arrangement of holes 8 is determined by the pattern 6. The arrangement and the dimensions of the holes 8 correspond exactly to the arrangement and the dimensions of the coating 5 for the completely vertical sides of the upright 4. The bottom layer 3 of the construction shown in fig. 4 and 5 can however also be supplemented by or replaced by a plurality of layers. Three layers (so-called trilayers) are possible in particular here.

It is known that a multilayer coated one-dimensional periodic grating can strongly filter color not only in reflection but also in transmission by constituting a Fabry-Perot resonator. In the case of three layers the following layering is particularly preferred: two semi-transparent metal layers with a dielectric spacer layer between them, or two high refractive layers with a low refractive layer between them. The following materials are conceivable for the metal layer: al, Ag, Pt, Pd, Au, Cu and alloys thereof. E.g. ZnS, ZnO, TiO₂，ZnSe，SiO，Ta₂O₅Or silicon is suitable as the high refractive layer. Providing SiO as a low refractive layer₂，Al₂O₃Or MgF₂。

Different processes or processes can be considered for the manufacturing method of the grating 1. The simplest manufacturing method is to first construct the dielectric carrier 2 with the elevations, for example pillars 4, arranged in a pattern 6 as a basic structure and then to plate it. It can be used for vertical evaporation and also can be used for oblique evaporation. It is important that the coating layers 5 are not coherent, i.e. are independent of each other.

For the production of the carrier 2, a moulding process can be used, so that inexpensive mass production is possible.

Metallic security elements with optically effective embossing are known from the prior art. These imprints are larger than the wavelength and are mostly present in the imprint lacquer covered with a metal layer. Such a security element is used in three different solutions: safety threads; security films, transmission strips (or patches) embedded in paper and provided with an active layer, wherein only the embossing lacquer coating the active layer is transferred onto the substrate; and a laminate film, a thin film provided with an active layer and bonded to a substrate.

The color configuration of these security elements can be carried out in the following manner: dyeing the carrier film or the embossing lacquer, printing the glazed color in sections below the embossing lacquer, printing the glazed color in sections above the metal layer, or embossing the diffractive element, wherein the dyeing is carried out by a first diffraction order (for example true-color holograms).

The above-described method for the security element has obvious drawbacks: dyed carrier films can only be used for safety threads and laminating films. Only one color can be visualized here. Furthermore, there are several transparent regions in the security element, in which no color should appear. Likewise, a dyed embossing lacquer does not allow the formation of security elements with distinct transparent regions, since the embossing lacquer is always present on all sides. Also a multicoloured design is not possible. The color of the conversion element can be designed, for example, with a glazed color underneath the embossing lacquer. This involves the problem that, on the one hand, the applied color must have a good adhesive bond with the impression lacquer. On the other hand, the adhesion to the carrier film printed thereon is only allowed to be small, so that the conversion process is not impaired. Another disadvantage is the low positioning accuracy of such printed colour layers with respect to effective embossing. Since the color printed layer and the stamp are not simultaneously formed in the same process, deviations are formed in the positions of the two layers from each other. The facets dyed within such a motif therefore tend to move by tens of millimetres relative to the embossed motif. Although the above-mentioned disadvantages are avoided when dyeing by means of diffractive structures, the color of the first order diffractive structures is strongly dependent on the viewing angle. The security element must be held at the correct angle relative to the light source to enable the colour corresponding to the design to be seen. Other colors appear under all other viewing conditions. Furthermore, the intensity of the first diffraction order of such a structure is mostly low compared to the reflection of the printed color.

The grating 1 described here achieves, for example, colouring of security elements in the zero diffraction order by means of a two-dimensional periodic sub-wavelength structure. All the disadvantages mentioned above can be avoided due to the large angular tolerances of the colors and the possibility of lateral structuring of the grating profile. The colored stamped features are color-intensive, can be positionally accurately configured relative to other security stamped features, and can exhibit a desired uniform color over a relatively wide viewing angle.

The embossed structures can either completely fill the area defined by the design and thus be colored, or they can be interlaced with other effective embossed structures in alternating small area sections by means of them, giving them a color. The coloured embossed structures may also be superimposed by other active embossed structures. This is possible, in particular, because many of the effective embossing structures used in security elements have individual structures with dimensions of between 700nm and 20 μm and can therefore be superimposed without interference with the colored embossing structures, which are typically at least 2-10 times smaller.

Finally, the above structure is integrated in the security film in the transparent zone. This feature appears differently colored when viewed under incident light from the front and back sides. The transparent elements may be of a different color to the reflection.

Table 1: parameters of two-dimensional periodic grating

Structure of the product	p[nm]	s[nm]	h[nm]	t[nm]
					a)	400	120	300	40
b)	240	117	300	40
					c)	330	167	300	40
d)	400	203	310	40
					e)	240	117	300	20
f)	240	117	300	50/45/50

All gratings detailed above were molded in UV lacquer on PET film, evaporated with aluminum, and finally coated with PET film. The refractive indices of the PET film and UV lacquer when visible were approximately equal to 1.56.

Fig. 7 to 10 show the transmission or reflection characteristics of the structures denoted by a to d in the above table. The reflection curve of the front side is denoted by R1, the reflection curve of the back side is denoted by R2, and the transmitted beam is denoted by T. These graphs each represent intensity as a function of wavelength. Aluminum having a layer thickness t of 40nm is used as the metal.

These gratings display different colors not only in reflection but also in transmission. The reflection at the front side is quite significantly different from the reflection at the back side. This situation appears most clearly in gratings c) and d). The grating c) has a red color on the front side and a blue color on the rear side. In contrast, transmission was observed from both sides to be blue. The grating d) then appears red on the front side, turquoise on the back side and green in transmission. The angular tolerance of the color is checked by reflectance measurements at different angles of incidence. Fig. 11 shows the reflection of grating e) for angles of incidence of 8 ° (curve 13), 30 ° (curve 15) and 45 ° (curve 14). The reflection maximum for this grating is in the blue and moves only slightly when changing the angle of incidence. Such a grating always appears blue for these different angles of incidence.

The above-mentioned mould-dependent colouring can be used in order to create coloured symbols or images. FIG. 12 shows three different profiles (p) of a grating_R、s_R、h_R)，(p_G、s_G、h_G) Or (p)_B、s_B、h_B) In the form of red, green and blue colored regions. These different colors can be caused by changing one or more profile parameters accordingly.

Three regions 17, 18, 19 are shown corresponding to the RGB sub-pixels and together forming a pixel 16. The respective profile in each region 17, 18, 19 ensures that the respective color red, green or blue results. At the same time, the color share of the respective RGB sub-pixels formed by the regions 17, 18 or 19 in the pixel 16 can be adjusted by means of the profile selection. Thereby giving the pixel 16 the desired color. Therefore, a true color image can be realized by color mixing caused in the pixel 16 by the ground color in the RGB sub-pixel regions 17, 18, 19. This structure has the advantage over conventional embossing techniques that it can be very precisely structured up to the micrometer level, which is advantageous in particular when the device is enlarged by means of ribs. The grating according to fig. 12 allows microscopic images in which the grating profile changes laterally in order to achieve a color or intensity contrast within the microscopic image. The structures described here are preferably suitable for this because their optical properties have a large angular tolerance, i.e. their color changes little when the angle of incidence changes. This characteristic is advantageous when combined with a microlens set, because the light seen by the viewer comes from different light paths with different angles of incidence.

Figure 13 shows the transmission or reflection characteristics of the grating according to configuration a.

If a grating with a three-layer coating according to fig. 6 is used, the optical properties according to fig. 14 and 15 are obtained. There is a sequence of 50nm ZnS, 45nm SiO₂And a 50nm ZnS three-layer coating 11. Such a grating displays an angle-independent blue color when reflected and a yellow color when transmitted. It corresponds to structure f in the above list.

It goes without saying that the terms "above" or "below" in the above description are merely exemplary and are to be understood as representing the interrelationship in the figures. Of course the structure can also be reversed as follows: the design of fig. 5 is based on, that is, the coating 5 is now located below the base layer 3. This has a particular effect if only the reflected beam contributes to the optical effect by placing the grating on an opaque material.

List of reference numerals

1 optical grating

2 support

3 bottom layer

4 column

5 coating layer

6 pattern (I)

7 dielectric

8 holes

9 front side

10 rear side

11. 12 multilayer structure

13. Curves 14 and 15

16 pixels

17. 18, 19 RGB sub-pixels

h height of column

thickness of t-plating

s₁Width of column

s₁Depth of column

p₁、p₂Period of time

Normal of surface A

E incident beam

R reflected beam

R1 beam reflected at the front side

R2 rear side reflected beam

T transmitted beam

Claims (16)

1. A security element for value documents, wherein the security element has a two-dimensional periodic color filter grating with a coherent, metallic, high-refractive-index base layer (3) defining a grating plane, and a two-dimensional regular pattern (6) of individual, metallic, high-refractive-index surface elements (5) on the base layer (3), the surface elements (5) each extending parallel to the grating plane and each being separated from the base layer (3) by an intermediate dielectric (4) by a distance (h) which is greater than the thickness of the base layer (3) and the surface elements (5), wherein the regular pattern (6) extends along at least two directions parallel to the grating planeHas a period (p) between 100nm and 800nm₁、p₂)。

2. Security element according to claim 1, wherein said period (p)₁、p₂) Between 200nm and 500 nm.

3. A security element according to claim 1, wherein the substrate (3) under each surface element (5) has a hole (8).

4. A security element according to claim 1, wherein the substrate (3) and the face element (5) comprise a material comprising: al, Ag, Cu, Cr, Si, Zn, Ti, Pt, Pd, Ta and alloys thereof.

5. Security element according to one of claims 1 to 4, wherein the intermediate dielectric (4) under each surface element (5) is designed as a pillar on which the surface element (5) is arranged.

6. A security element according to claim 5, wherein the intermediate dielectric (4) has a refractive index between 1.4 and 1.6.

7. A security element according to one of claims 1 to 4, wherein said grating is embedded in an embedding dielectric (7).

8. A security element according to claim 7, wherein the buried dielectric (7) has the same refractive index as the intermediate dielectric (4).

9. Security element according to one of claims 1 to 4, wherein the regular pattern (6) of surface elements (5) has a surface filling factor of 0.15 to 0.85.

10. Security element according to one of claims 1 to 4, wherein the regular pattern (6) of surface elements (5) has a surface filling factor of 0.3 to 0.7.

11. A security element according to one of claims 1 to 4, wherein the distance (h) over which the surface element (5) lies in the plane of the grating varies parallel to the plane of the grating.

12. A security element according to one of claims 1 to 4, wherein the grating has a dielectric carrier layer (2) whose upper side is structured as a pillar structure with side faces extending substantially perpendicularly to the grating plane, and the high refractive layer (3, 5) is applied on the dielectric carrier layer (2) in such a way that the side faces are not completely coated.

13. Security element according to one of claims 1 to 4, wherein, for generating a colored image information, the grating has a locally changed area fill factor and/or a locally changed distance between the area elements (5) and the underlayer (3).

14. A security element according to claim 13, wherein the grating provides a microscopic image structure to the moire magnification device.

15. A value document precursor not yet suitable for circulation, having a security element according to any of the preceding claims.

16. A value document precursor not yet suitable for circulation according to claim 15, wherein the security element spans a transparent area or cut-out.