Patent Description:
Security devices are being used more and more to protect currency and other valuable documents such as passports, drivers' licenses, green cards, identity cards and the like. These security devices are also used to protect commercial products such as pharmaceuticals, cosmetics, cigarettes, liquor, electronic media, wearing apparel, toys and spare parts for automobiles and aircraft from counterfeiting. In fact, it is estimated that counterfeit articles now comprise between <NUM>% and <NUM>% of world trade. Holograms attached to such articles have been the traditional method to foil counterfeiters.

Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object.

<CIT> discloses a security article having Chromagram™ thereon. The Chromagram™ provides both color shifting and holographic effects to the viewer. In the '<NUM> patent an organic substrate stamped with a holographic grating or pattern is coated with a color shifting multilayer film.

<CIT>, discloses a more complex type of Chromagram™ wherein patterning is shown. In some regions, holographic effects are shown, and in other regions only color shifting effects are visible. In Phillips '<NUM> patent a multilayer thin film filter is disclosed an organic dielectric layer serving as a spacer layer in a Fabry-Perot structure. The dielectric has embossed regions of varying thicknesses wherein the thickness within a region is substantially uniform. Each different region of a different thickness produces a different color shift. The size of one of the embossed adjacent regions is such that the color of said one region is uniform and cannot be seen by a human eye as different in color from the uniform color of an adjacent region thereto, and wherein the color within a region can be seen with magnification of at least <NUM>:<NUM>. Phillips' teaches a Fabry-Perot device with a variable thickness dielectric layer by embossing the dielectric material to various thicknesses. Since the dielectric in regions a, b, and c as shown in <FIG> of the '<NUM> patent are purposefully embossed with different thicknesses, light reflecting back to the viewer after impinging upon the reflector will be three different distinct colors. However due to the small size of the regions a, b, and c, the eye will tend to integrate and if the pixel or region defined by (a) through (d) inclusive can be seen; only a single color will be perceived. With sufficient magnification, the individual regions (a), (b), and (c) will be seen and different colors will be perceived.

Another United States patent application which discloses diffraction gratings with color shifting coatings but deviates from the teaching of <CIT>. United States Patent '<NUM> appears to deviate from the teaching of Phillips in that a decoupling layer is taught as way in which to separate the diffraction grating effects from the color shifting effects. Holmes suggests placing a decoupling layer between the relief structure and the thin film reflection filter, which is described to be a thin film reflection filter.

In all of these aforementioned security structures, conventional application of the coating is suggested, for example by vacuum deposition to yield conforming layers.

The prior art teaches first stamping a substrate, and subsequently applying the coating layers required to create the desired patterns of reflective and color shifting coatings.

This invention deviates from the prior art teaching by using conforming coatings with non-conforming coatings on substrates having structures thereon. In preferred embodiments the structures stamped or formed upon the substrate layer are so small, for their effects to be seen, magnification is required, however in other less preferred embodiments the structures may be large enough that they can be seen without magnification when coated. By way of example logos and other readable discernible indicia are provided on these substrates and are highlighted by providing thin film coatings that contrast particular regions.

This invention provides a thin film structure that is coated on a substrate wherein the dielectric spacer layer has a varying thickness. The provision of a dielectric layer with a varying thickness has been disclosed not only by<CIT> but also much earlier in <CIT> Shaw et al disclose applying heat variably to create a dielectric layer of varying thickness.

In contrast to the prior art which uses a stamped substrate as a spacer layer coated on one side with a reflector and on another side with an absorbing layer, an embodiment of this invention uses non-conforming dielectric layer coated on a same side of a microstructured substrate as a reflective layer and absorbing layer. Therefore the Fabry-Perot structure is supported by the substrate. This provides numerous advantages. One advantage is that the coating can be removed from the substrate if coated with a release layer. Furthermore this coating can be made into shaped flakes if carefully removed from the substrate.

The Fabry-Perot structure of this invention provides different color shifting regions adjacent to one another which preferably differ in their color from one another by at least a delta E value of <NUM>.

It is an object of this invention to provide a device, which exhibits different color shifting regions, visible with magnification, wherein adjacent color shifting regions provide a color shift between two distinct different colors due to the dielectric spacer layer having a varying thickness.

It is an object of this invention to provide a substrate having relief structures across its surface so that a cross section thereof has a varying thickness, and to mirror that varying thickness by applying a non-conforming layer filling depressions, valleys and troughs with a dielectric material so as to provide a Fabry-Perot structure having a spacer layer which provides color shift differences corresponding to the thickness of the substrate.

It is an object of this invention to provide at least one conforming layer and a non- conforming layer to fabricate a Fabry-Perot color-shifting filter, and wherein a substrate supporting the Fabry-Perot filter is purposefully embossed with a predetermined pattern to provide encoding that will form color-shifting indicia within the filter.

It is a further object of this invention to provide a flake having a non-conforming dielectric layer with at least one conforming layer and another conforming or non-conforming layer, wherein the flake is a color shifting device.

In accordance with the invention, there is provided a color shifting security device according to claim <NUM>.

In a preferred embodiment of the invention the first layer, or the second layer, has a substantially uniform thickness, which varies by no more than <NUM>%, and/or the difference in thickness of cross-section of the dielectric non conforming layer is more than <NUM>/<NUM> wavelengths of visible light and less than <NUM> quarter wavelengths of visible light.

In a particular embodiment the regions of the device corresponding to particular microstructures form visible indicia that can be seen with magnification, and wherein the height or depth of some of the microstructures are at least <NUM>.

Optionally, at least some of the particular microstructures form valleys, which in cross-section form flat-bottomed valleys.

Optionally, peaks of the particular microstructures in cross-section are flat-topped structures.

The color shifting security device comprises a substrate, wherein the substrate has microstructures corresponding to the microstructured surface of the first layer.

The microstructures define a logo or discernible indicia.

In accordance with another aspect of the invention a method is provided of fabricating a security device according to claim <NUM>.

In a particular embodiment the aforementioned color shifting structure is a flake.

Optionally, the Fabry-Perot cavity forms a flake after being removed from a substrate.

Optionally, the color shifting security device, further comprises a substrate, wherein the substrate and layers thereon form a foil.

Optionally, the dielectric non-conforming layer is a discontinuous layer having gaps between regions of different thickness of dielectric material.

The dielectric non-conforming layer fills in grooves within the microstructured surface to form a planar surface over a continuous region of the microstructured surface; and the second absorbing covers at least a part of the continuous region.

Optionally, the microstructures have selectively chosen depths, such as in cross-section flat-bottomed valleys, or protuberances in the form of upstanding features, such as in cross-section flat-topped structures.

Optionally, microstructures have a number of distinct levels or depths.

Optionally, the grooves are of two different depths.

Optionally, the Fabry-Perot cavity forms a flake.

Optionally, the first layer, or the second layer, has a substantially uniform thickness, which varies by no more than <NUM>%.

Optionally, a difference in thickness of a cross-section of the dielectric non-conforming layer is more than <NUM>/<NUM> wavelengths of visible light and less than <NUM> quarter wavelengths of visible light.

Optionally, the color shifting security device further comprises a second dielectric layer.

The second dielectric layer conforms to the microstructured surface.

The second dielectric layer is disposed between the first deposited layer having a microstructured surface and the non-conforming dielectric layer.

Optionally, the non-conforming dielectric layer includes a first material that conforms to the microstructured surface and a second non-conforming material that fills in grooves within the first material.

Optionally, the non-conforming dielectric layer is a discontinuous layer having gaps between regions of different thickness of dielectric material.

Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:.

The examples of <FIG>, <FIG>, <FIG> and <FIG> are not according to the invention and are present for illustration purposes only.

The invention is related to the use of thin dielectric non-conforming layers on microstructured surfaces allowing for the manufacturing of devices having micro areas of different color shifting. The different colors are obtained by thin film interference when the thickness of the dielectric layer varies in different regions. Different color shifting refers to a different range of colors; for example due to the thickness of the spacer layer in different regions of the device, one region may shift from orange to brown and another region may shift from gold to green.

Conforming deposited layers are obtained when the species in the vapor phase condenses as a solid. This is the case of most of the metals and their compounds; when oxides, nitrides, carbides, fluorides, combinations, etc. are deposited by standard vacuum physical vapor deposition, sputtering and evaporation, or by chemical vapor deposition.

Once the species in the vapor phase condenses on a substrate, there is not enough mobility of the condensed species in the form of mobile atoms, radicals or molecules. Therefore the condensed species will be fixed on the surface of the substrate following the original roughness of the substrate.

In contrast, a non-conforming layer will act similar to a layer of water resting upon a surface, filling any roughness of the surface to create a planar surface independently of the roughness of the surface. When water is solidified, for example by freezing in optimal conditions when the layer is not disturbed during the freezing process, the solid layer will present the smoothness of the original water liquid layer. Water will fill in any voids and will yield a planar upper surface.

Although the illustrative example of water allows one to envisage how a non- conforming layer behaves, other materials, in particular some selected monomers exhibiting similar behaviour, provide the smoothing or planarizing properties in the liquid state and can be solidified by a post polymerization stage by ultra-violet (UV) or electron radiation. Selected light transmissive monomers having preferred properties such as a suitable refractive index can be used as a spacer layer in a Fabry-Perot filter.

To deposit monomers they are heated within a container so as to produce a vapor. When the vapor makes contact with a cooler surface in proximity it condenses upon the cooler surface. Therefore, non-conforming layers are obtained when a monomer in the gas phase is brought into contact with a cooled substrate whereby the gas phase condenses forming a liquid layer. The liquid layer supported by the substrate is subsequently cured, producing the polymerization of the liquid monomer into a solid layer.

The monomer can be evaporated by heating it in a reservoir with an aperture or nozzle used to build the desired pressure of the monomer vapor before it expands in the vacuum chamber. If the vapor pressure of the monomer is not high enough to produce a gas stream directed at the substrate, an inert gas can be introduced into the liquid monomer. In an alternative embodiment, the liquid monomer can be directly sprayed in a hot reservoir to be instantaneously evaporated to achieve flash evaporation. Care must be taken to ensure that the temperature of the reservoir is low enough to avoid degradation of the monomer or its thermal polymerization.

Although evaporation is the preferred method of depositing the dielectric monomer, printing, painting, extrusion, spin-off, or the use of a doctoring-blade, may be considered; however, often these technologies have the tendency to form layers that are too thick to create interference for visible wavelengths of light. Various monomers and/or oligomers can be used as non-conforming layers. By way of example, the non-confirming layer can be formed using any of the following materials: epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, amine modified polyether acrylates, acrylic acrylates and miscellaneous acrylate oligomers.

This invention provides a method for fabricating one or more thin-film Fabric-Perot interference devices upon a microstructured substrate that will exhibit a color change when irradiated with visible light when the angle of incidence or viewing angle changes.

Referring now to prior art <FIG> a three-layer Fabry-Perot cavity is shown. The substrate <NUM> has deposited thereon a conforming layer 101a of a highly reflective material such as Al. Deposited on the aluminum layer 101a is a dielectric conforming layer 102a. A conforming absorber layer 103a is subsequently deposited on the dielectric layer 102a. Using conventional vacuum coating techniques results in a thin film optically variable filter upon a substrate wherein each layer has a substantially uniform thickness. Notably, since the surface of the substrate is flat, each layer will be a uniform thickness whether conforming layers or nonconforming layers are deposited, providing a same optical effect when applied to a planar surface such as that in <FIG>. However, the optical effects obtained for conforming or non-conforming layers will be different when the substrate has a microstructured surface. Non-conforming layers will fill in voids where conforming layers simply conform to the microstructured surface so that they are substantially uniform in thickness.

In operation, a thin-film Fabry-Perot filter functions as a color changing element; as the angle of light incident upon the cavity is varied between the light source and the viewer, the color varies as a function of the path length through the dielectric layer varying with the change in angle.

Turning now to <FIG> a substrate is shown in cross-section where microstructures <NUM> pointing upward from the substrate are shown, and wherein the height of the upstanding structures is uniform. A three dimensional perspective isometric view is shown in <FIG> and a top view is shown in <FIG>.

<FIG> illustrate an example wherein the microstructures within the substrate <NUM> are in the form of grooves <NUM> of varying depth within the substrate.

<FIG> show a substrate <NUM> wherein a grating formed of grooves <NUM> of a first depth are bound by deeper framing grooves <NUM> within the substrate.

<FIG> show a cross section of a substrate coated with a coating material where the layer has been grown atom by atom by conventional vacuum coating processes as evaporation and sputtering. The layer conforms to the substrate following the original microstructure of the surface. If for example a <NUM> layer RI D/A is coated, the same color by thin film interference will be seen everywhere in the substrate since the thickness of the dielectric is constant as shown in <FIG>.

Referring now to prior art <FIG> a substrate <NUM> having embossed grooves <NUM> and <NUM> of varying depth shows a reflector layer <NUM> of a first uniform thickness, a dielectric layer <NUM> of a second uniform thickness, and an absorber layer <NUM> of a third uniform thickness coated over the substrate <NUM> wherein of the layers are conforming layers.

In <FIG> the same substrate as shown in <FIG> is used however one of the coating layers in <FIG> is non-conforming providing a functionally differing device from <FIG>. Turning now to <FIG> a substrate <NUM> is shown having a conforming reflector layer <NUM> of uniform thickness coated directly thereon. Upon the reflector layer is a non-conforming coating of dielectric material, which fills in the grooves within the reflector coated substrate and has an upper substantially planar layer. As a result the dielectric layer <NUM> has a varying thickness, in cross-section, as shown. Two different thicknesses result when the dielectric layer is coated over substrate <NUM> due to the two different depths within the microstructured substrate <NUM>. The two different depths of the dielectric spacer layer provide two different color shifting regions, where the color shifts from a different first color, to a different second color in the regions of different thickness. For a perceivable color difference to be seen in the two regions of different thickness, a thickness difference in the spacer or dielectric layer, is required. As can be seen in <FIG> the thickness difference in the spacer layer is considerably larger than the combined thickness of the adjacent two layers <NUM> and <NUM>. An absorber layer <NUM> having a substantially uniform thickness is shown over the dielectric layer <NUM>. The absorber layer <NUM> could be a conforming layer or a non-conforming layer since it is a planar layer applied onto a planar surface. However, preferably, a conforming absorber layer is used, typical of conventional color-shifting filters. The thickness of the dielectric layer can be selectively controlled by providing microstructures having selectively chosen depths or protuberances in the form of upstanding features, as the dielectric layer essentially fills in voids resulting in a varying of its thickness. In <FIG> color shifting regions <NUM> having a first color shifting range of colors and color shifting regions <NUM> have a second color shifting range of colors. Typical thickness ranges for the absorber layer would be <NUM> Angstroms to <NUM> Angstroms depending upon which metal was selected. The reflectivity of the reflector layer is preferably at least <NUM>% to provide an adequate visual effect from the device and the dielectric spacer layer could vary be as much as <NUM>.

When a non-conforming or conforming dielectric is applied to a single level macrostructure surface such as that of <FIG>, two different colors will be produced by thin film interference corresponding to the different thickness of the planarizing dielectric layer as the angle of incidence increases. Notice that the reflector and absorber layers applied are conforming layers. Since a dielectric polymeric layer tends to have an index of refraction in between <NUM> and <NUM>, the thin-film interference will produce colors that shift from high to low wavelengths as the angle of illumination increases.

Advantageously, a release layer can be applied in between the substrate and the deposited layers with the intention to strip off the multilayer to make micro multi-color shifting microstructured pigment flakes. The release layer can also be used to transfer the multilayer to another object. If the device is intended to make thread, yam, or foils it may not require the use of release layers. Such flakes are typically less than <NUM> or equal thereto, across a longest length. The difference shown in the figures between the two dielectric thicknesses are exaggerated. The aspect ratio for the microstructured character is <NUM>-<NUM> of depth for a line width that is typically <NUM>-<NUM>.

The microstructure within the substrate can represent symbols, logos, grating, frames, peaks/valleys, etc. as shown in <FIG>. Advantageously the color shifting coating provides a way in which these features, such as logos, etc., can be enhanced.

Turning now to <FIG> grooves in substrate <NUM> are of two different depths. When the non-conforming dielectric layer <NUM> is deposited over the conforming reflector layer <NUM> and an absorbing layer <NUM> is applied thereover, the resulting structure is a Fabry-Perot color-shifting filter having three distinct ranges of color shifting. The non-conforming layer provides a planarizing smoothing effect upon which layer <NUM> is deposited conforming to this planarized layer. As the number of distinct levels or depths within the microstructure increases the number of ranges of color shifting increases accordingly.

<FIG> illustrates an embodiment of the invention wherein a microstructured substrate <NUM> is coated with a conforming reflector layer <NUM> and where conforming and non-conforming dielectric layers 902a and 902b respectively are used adjacent to one another in a same device. A planar absorber layer <NUM> is coated over the non-conforming dielectric layer 902b. This planar layer <NUM> could be a conforming or a non-conforming layer since it is being applied to a planar surface. In this device three different color ranges are seen due to the three thicknesses of the combined dielectric layers. As mentioned previously, generally non-conforming polymeric dielectric layers have a lower refractive index than standard inorganic oxides layers.

By using a judiciously selected combination of a high refractive index inorganic dielectric with a lower refractive index polymer dielectric further control the color shifting properties can be attained.

<FIG> exemplifies a microstructured foil.

Turning now to the device of <FIG> shown in cross section, the microstructure substrate <NUM> is shown coated with a reflector layer <NUM>, which is coated with a conforming first dielectric layer 1002a. A second non-confirming polymeric layer 1002b is coated and only fills in trenches or grooves within the coated substrate <NUM>. Absorber layer <NUM> is coated as a top layer forming together with the other coated layers a color-shifting filter. In practice this could be achieved by eliminating the top of the polymeric dielectric of <FIG>, for example by ion bombarding under vacuum until reaching suitable level of the inorganic oxide layer prior to the deposition of the absorber layer.

In <FIG> a substrate <NUM> having protuberances or upstanding structures is shown. This example lends itself more to applying a release layer than the previously described structures. If a release layer is applied, it is first applied prior to depositing the reflector layer <NUM>, so that the reflector layer and subsequent deposited layer can together be released from the substrate. The organic non-conforming dielectric layer <NUM> is deposited to a level lower than the higher areas that will be used as braking points to produce the shaped flakes. Only the thin layers corresponding to the reflector <NUM> and absorber <NUM> will be in the top of these areas.

After separating the multilayer from the substrate and forming shaped flakes, these flakes will have different properties when viewed from different sides. When viewed from the side having a reflector layer, the flakes will simply be reflective. However on the opposite side, a viewer with magnification would see the logos or symbols with a color shifting exhibited surrounded by a background of a different color. From the reflective side logos may be discernible however the color will correspond to that of the reflector layer.

If the absorber layer is not applied, the top of the higher areas have a thin metal layer exposed surrounded by a dielectric layer. In this instance, the top areas can be used as seed point to grow preferentially other layers, for example one can perform electroplating using the exposed metallic layer as electrodes. Such devices can be used for other applications such as for sensors where micro exposed metallic layers are necessary.

In <FIG> the multilayer Fabry-Perot filter is formed of a five-layer structure with layers A/D/R/D/A. Since the reflector layer <NUM> is shown as a central layer, color shifting will be seen from both sides of this flake after it is released from the substrate <NUM>. Upon the substrate is a release layer, not shown and a first absorber layer 1203a. Upon the first absorber layer is a first non-conforming dielectric layer 1202a. The reflector layer <NUM> is shown deposited upon the first dielectric layer 1202a. A second non-conforming dielectric layer 1202b is deposited over the reflector layer <NUM> and a conforming 2nd absorber layer 1203b is deposited over the second non-conforming dielectric layer 1202b. After releasing the multilayer, the shaped flakes when broken along the breaking lines, exhibit on side <NUM> the Symbol <NUM> with a non-shifting color corresponding to Absorber /Reflector and symbol <NUM> corresponding to a color shifting (CS4) from the multilayer Absorber/Dielectric/Reflector surrounded by another color shifting background (CS3).

When viewed on side <NUM> the flake will show a color-shifting (CS2) symbol <NUM> with a background of a different color (CS1). Symbol <NUM> will not be seen due to the presence of the opaque reflector layer. Since these flakes are small and below resolution that can be seen with an unaided eye, magnification would be required to see these aforementioned features.

The example shown in <FIG> differs to the example shown in <FIG> in the optical design used to create the thin-film interference. In <FIG> a microstructured substrate <NUM> is shown having a first conforming absorber layer <NUM> instead of a reflector layer. A non conforming dielectric layer <NUM> is coated over layer <NUM> and a conforming 2nd absorber layer <NUM> is coated over the dielectric layer. Thin film interference is obtained by this three-layer Absorber/Dielectric/ Absorber design. Such optical designs are semi transparent. If coated on a substrate with the features up shown in a previous example with logos with a single height, the shaped flakes will show the symbols with a different color than their background in both sides. If the symbols have more than one height in cross-section, different areas of the logo will show different colors.

In all instances, the variation in the thickness of the dielectric layer is much greater than the thickness of each of the two layers adjacent the dielectric layer.

Claim 1:
A color shifting security device comprising:
a first layer (<NUM>) having a microstructured surface;
a planar second layer (<NUM>);
a dielectric conforming layer (902a) and a dielectric non-conforming layer (902b) disposed between the first layer (<NUM>) and the second layer (<NUM>), the dielectric conforming layer (902a) having a surface contacting and complementary with the microstructured surface of the first layer (<NUM>), the dielectric non-conforming layer (902b) is a low refractive index polymer layer which is combined with a high refractive index inorganic dielectric material, and formed from an infill non-conforming dielectric material able to fill in grooves within the dielectric conforming layer (902a) to form the non-conforming dielectric layer (902b) and to form a planar surface over a continuous region of the microstructured surface; and
a substrate (<NUM>) supporting the first layer (<NUM>), wherein the substrate (<NUM>) has microstructures corresponding to the microstructured surface of the first layer (<NUM>),
wherein the first layer (<NUM>) is a reflector layer, wherein the second layer (<NUM>) is an absorbing layer, and wherein a cross section of the dielectric non-conforming layer (902a) has a varying thickness such that at least one region is substantially thicker than a thinner adjacent region of said layer, and wherein a visible color difference is seen when viewing the filter through the one region and the adjacent region from a same location simultaneously when light is incident upon the filter,
wherein the substrate (<NUM>) is respectively coated with the first layer (<NUM>), the conforming dielectric layer (902a), the non-conforming dielectric layer (902b), and the second layer (<NUM>),
wherein the first layer (<NUM>), the second layer (<NUM>) and the dielectric non-conforming layer, together form a Fabry-Perot cavity, and
wherein the microstructures define a logo or discernible indicia.