Optical security device comprising a layer having a microstructured surface and a dielectric layer filling in or complementary to the microstructured surface

Abstract

A color shifting security device has a Fabry-Perot type structure wherein a dielectric layer is disposed between a reflector and an absorbing layer. The absorber and reflector layers may be conforming and the dielectric layer therebetween is non-conforming, filling the regions in the micro structured adjacent absorbing or reflecting layer, at least one of which has a microstructure therein or thereon. By having the dielectric layer not conform to the microstructure it is next to, its thickness varies in cross section, which allows for different colors to be seen where the thickness varies.

Claims

1. An optical device comprising: a first absorbing or reflecting layer having a microstructured surface; a second absorbing or reflecting layer having a lower surface; and a first dielectric layer disposed between the first absorbing or reflecting layer and the second absorbing or reflecting layer, wherein the first dielectric layer has a first surface complementary with the microstructured surface and a second surface complementary with the lower surface, wherein the first dielectric layer at least partially fills in valleys within the microstructured surface, wherein a first region of the first dielectric layer has a first thickness, and wherein a second region of the first dielectric layer has a second thickness different than the first thickness.

2. The optical device of claim 1, wherein the second surface of the first dielectric layer is substantially planar.

3. The optical device of claim 1, further comprising: a second dielectric layer.

4. The optical device of claim 3, wherein the second dielectric layer conforms to the microstructured surface.

5. The optical device of claim 3, wherein the second dielectric layer is disposed between the first absorbing or reflecting layer and the first dielectric layer.

6. The optical device of claim 3, wherein the second dielectric layer is contacting and complementary with the microstructured surface.

7. The optical device of claim 1, wherein the first dielectric layer includes a first material that conforms to the microstructured surface and a second non-confirming material that fills in grooves within the first material.

8. The optical device of claim 1, wherein a visible color difference is present when viewing the optical device through the first region of the first dielectric layer and the second region of the first dielectric layer from a same location, simultaneously, when light is incident upon the optical device.

9. The optical device as defined in claim 1, wherein the first absorbing or reflecting layer, the second absorbing or reflecting layer, and the first dielectric layer together form a Fabry-Perot cavity.

10. The optical device of claim 1, wherein the first absorbing or reflecting layer or the second absorbing or reflecting layer has a substantially uniform thickness that varies by no more than 20%.

11. The optical device of claim 1, wherein a difference in thickness of a cross-section of the first dielectric layer is more than wavelengths of visible light and less than 8 quarter wavelengths of visible light.

12. The optical device of claim 1, wherein a valley, of the valleys of the microstructured surface, in cross-section is a flat-bottomed valley.

13. The optical device of claim 1, wherein a peak, of the microstructured surface, in cross-section is a flat-topped peak.

14. The optical device of claim 1, wherein the second absorbing or reflecting layer is a reflector layer.

15. The optical device of claim 1, wherein the first absorbing or reflecting layer is an absorbing layer.

16. The optical device of claim 1, wherein the second absorbing or reflecting layer is an absorbing layer.

17. An optical device comprising: a Fabry-Perot cavity comprising: a first layer having a microstructured upper surface, wherein the first layer is one of an absorbing layer or a reflecting layer; a second layer, wherein the second layer is one of an absorbing layer or a reflecting layer; and a dielectric layer having an upper surface and a lower surface disposed between the microstructured upper surface of the first layer and the second layer, wherein the lower surface of the dielectric layer conforms to the microstructured upper surface of the first layer, wherein the dielectric layer at least partially fills in valleys within the microstructured upper surface, and wherein a first region of the dielectric layer has a first thickness that is different from a second thickness of a second region of the dielectric layer.

18. The optical device of claim 17, wherein the first layer is the absorbing layer.

19. An optical device comprising: one or more deposited layers forming a microstructure, wherein at least one of the deposited layers is a reflecting or absorbing layer; a dielectric material filling in grooves within the microstructure to form a planar surface over a continuous region of the microstructure; and an absorbing or reflective cover layer covering at least a part of the continuous region.

20. The optical device of claim 19, wherein the dielectric material is an infill non-conforming material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:

(2) FIG. 1 is cross-sectional view of a prior art three-layer Fabry-Perot cavity.

(3) FIG. 2a is cross-sectional view a prior art substrate having frames in relief as upstanding walls extending from the substrate surface and an upstanding logo.

(4) FIG. 2b is an isometric view of the prior art structure of FIG. 2a.

(5) FIG. 2c is a plan view of the prior art structure shown in FIG. 2b.

(6) FIG. 3a is a cross-sectional view of a prior art substrate having grooved frames and a grooved logo.

(7) FIG. 3b is an isometric view of the prior art structure of FIG. 3a.

(8) FIG. 3c is a plan view of the prior art structure shown in FIG. 3b.

(9) FIG. 4a is a cross-sectional view of a prior art substrate having grooved frames and a grooved grating across its surface.

(10) FIG. 4b is an isometric view of the prior art structure of FIG. 4a.

(11) FIG. 4c is a plan view of the prior art structure shown in FIG. 4b.

(12) FIG. 4d is a cross-sectional view of a substrate having a squared array of peaks and valleys.

(13) FIG. 4e is an isometric view of the prior art structure of FIG. 4d.

(14) FIG. 4f is a plan view of the prior art structure shown in FIG. 4d.

(15) FIG. 5a is a prior art cross-section of a microstructured foil wherein symbols are a same depth and wherein the coating is a uniform thickness.

(16) FIG. 5b is a prior art cross-section of a microstructured foil wherein symbols are a same depth and wherein the coating is a non-uniform thickness.

(17) FIG. 6 is a prior art microstructured substrate having grooves therein coated with three conforming layers forming a Fabry-Perot color shifting coating.

(18) FIG. 7 is a cross-sectional view of a microstructured structure having a conforming reflector layer, a non-conforming dielectric layer and an absorber layer together forming a Fabry-Perot structure in accordance with this invention wherein two different alternating color shifting regions are provided due to the difference in thickness of the non-conforming dielectric layer.

(19) FIG. 8 is a cross-sectional view of a microstructured structure having a conforming reflector layer, a non-conforming dielectric layer and an absorber layer together forming a Fabry-Perot structure in accordance with this invention wherein three different alternating color shifting regions are provided due to the difference in thickness of the non-conforming dielectric layer.

(20) FIG. 9 is a cross-sectional view of an embodiment of the invention wherein a Fabry-Perot structure in the form of a microstructured foil has a conforming dielectric layer adjacent to a non-conforming dielectric layer.

(21) FIG. 10 is a cross-sectional view of an embodiment of the invention wherein a conforming and non-conforming dielectric layers are used and wherein an upper surface of the two dielectrics are at a same level such that a deposited absorber layer thereover is planar.

(22) FIG. 11 is a cross-sectional view of a microstructured substrate having a release coat for forming flakes in accordance with this invention.

(23) FIG. 12 is a cross-sectional view of a coating for forming a shaped flake in accordance with this invention wherein conforming and non-conforming layers are used and wherein a reflective layer is a central layer such that the flake is color shifting when viewed from either side.

(24) FIG. 13 is cross-sectional view of an alternative embodiment wherein a reflector layer is not required and is replaced with an additional absorber layer wherein color shifting is seen from both sides.

DETAILED DESCRIPTION

(25) 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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 behavior, 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.

(30) 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. In accordance with this invention, the liquid layer supported by the substrate is subsequently cured, producing the polymerization of the liquid monomer into a solid layer.

(31) 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.

(32) 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 in this invention. 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.

(33) 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.

(34) Referring now to prior art FIG. 1 a three-layer Fabry-Perot cavity is shown. The substrate 100 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 non-conforming layers are deposited, providing a same optical effect when applied to a planar surface such as that in FIG. 1. 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.

(35) 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.

(36) Turning now to FIG. 2a a substrate is shown in cross-section where microstructures 201 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. 2b and a top view is shown in FIG. 2c.

(37) FIGS. 3a through 3c illustrate an embodiment wherein the microstructures within the substrate 300 are in the form of grooves 301 of varying depth within the substrate.

(38) FIGS. 4a through 4c show a substrate 400 wherein a grating formed of grooves 401 of a first depth are bound by deeper framing grooves 402 within the substrate.

(39) FIGS. 5a and 5b 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 3 layer R/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. 6.

(40) Referring now to prior art FIG. 6 a substrate 600 having embossed grooves 612 and 614 of varying depth shows a reflector layer 601 of a first uniform thickness, a dielectric layer 602 of a second uniform thickness, and an absorber layer 603 of a third uniform thickness coated over the substrate 600 wherein of the layers are conforming layers.

(41) A first embodiment of the invention is shown in FIG. 7 wherein the same substrate as shown in FIG. 6 is used however one of the coating layers in FIG. 7 is non-conforming providing a functionally differing device from FIG. 6. Turning now to FIG. 7 a substrate 700 is shown having a conforming reflector layer 701 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 702 has a varying thickness, in cross-section, as shown. Two different thicknesses result when the dielectric layer is coated over substrate 700 due to the two different depths within the microstructured substrate 700. 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. 7 the thickness difference in the spacer layer is considerably larger than the combined thickness of the adjacent two layers 703 and 701. An absorber layer 703 having a substantially uniform thickness is shown over the dielectric layer 702. The absorber layer 703 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. 7 color shifting regions 706 having a first color shifting range of colors and color shifting regions 708 have a second color shifting range of colors. Typical thickness ranges for the absorber layer would be 20 Angstroms to 150 Angstroms depending upon which metal was selected. The reflectivity of the reflector layer is preferably at least 20% to provide an adequate visual effect from the device and the dielectric spacer layer could vary be as much as 800 nm.

(42) When a non-conforming or conforming dielectric is applied to a single level macrostructure surface such as that of FIG. 1, 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 1.5 and 1.7, the thin-film interference will produce colors that shift from high to low wavelengths as the angle of illumination increases.

(43) 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, yarn, or foils it may not require the use of release layers. Such flakes are typically less than 100 mm 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 100-500 nm of depth for a line width that is typically 1-5 um.

(44) The microstructure within the substrate can represent symbols, logos, grating, frames, peaks/valleys, etc. as shown in FIGS. 2a through 3c. Advantageously the color shifting coating provides a way in which these features, such as logos, etc., can be enhanced.

(45) Turning now to FIG. 8 a second embodiment of the invention is shown wherein grooves 808 and 809 in substrate 800 are of two different depths. When the non-conforming dielectric layer 802 is deposited over the conforming reflector layer 801 and an absorbing layer 803 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 803 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.

(46) FIG. 9 illustrates an embodiment of the invention wherein a microstructured substrate 900 is coated with a conforming reflector layer 901 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 903 is coated over the non-conforming dielectric layer 902b. This planar layer 903 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. 9 exemplifies a microstructured foil.

(47) Turning now to the device of FIG. 10 shown in cross section, the microstructure substrate 1000 is shown coated with a reflector layer 1001, 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 1001. Absorber layer 1003 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. 9, for example by ion bombarding under vacuum until reaching suitable level of the inorganic oxide layer prior to the deposition of the absorber layer.

(48) An alternative embodiment of the invention described heretofore is shown in FIG. 11. In this instance a substrate 1100 having protuberances or upstanding structures is shown. This embodiment 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 1101, so that the reflector layer and subsequent deposited layer can together be released from the substrate. The organic non-conforming dielectric layer 1102 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 1101 and absorber 1103 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.

(49) In an alternative embodiment 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.

(50) An embodiment similar to that shown in FIG. 11 is shown in FIG. 12 however the multilayer Fabry-Perot filter is formed of a five-layer structure with layers A/D/R/D/A. Since the reflector layer 1201 is shown as a central layer, color shifting will be seen from both sides of this flake after it is released from the substrate 1200. 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 1201 is shown deposited upon the first dielectric layer 1202a. A second non-conforming dielectric layer 1202b is deposited over the reflector layer 1201 and a conforming 2.sup.nd 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 1 the Symbol 2 with a non-shifting color corresponding to Absorber/Reflector and symbol 1 corresponding to a color shifting (CS4) from the multilayer Absorber/Dielectric/Reflector surrounded by another color shifting background (CS3).

(51) When viewed on side 2 the flake will show a color-shifting (CS2) symbol 2 with a background of a different color (CS1). Symbol 1 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.

(52) The embodiment shown in FIG. 13 differs to the embodiment shown in FIG. 11 in the optical design used to create the thin-film interference. In FIG. 13 a microstructured substrate 1300 is shown having a first conforming absorber layer 1301 instead of a reflector layer. A non-conforming dielectric layer 1302 is coated over layer 1301 and a conforming 2.sup.nd absorber layer 1303 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 embodiment 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.

(53) 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.

(54) By depositing a non-conforming dielectric spacer layer in a Fabry-Perot structure, this invention allows for the fabrication of filters which have a varying thickness spacer layer and wherein the thickness can be precisely controlled. This allows for a single layer be it either continuous or segmented to provide different color shifting in different regions across the filter as function of the spacer layer thickness. Either flakes or foil can be made. Typically the non-conforming dielectric layer is coated over a conforming layer and covered with a conforming layer, however the dielectric layer could be coated over a non-conforming layer or may be covered with a non-conforming layer.