Patent ID: 12259569

DETAILED DESCRIPTION OF THE INVENTION

Preferably, an average diagonal of the smallest zones of a pattern are smaller than 1 mm, more preferred smaller than 0.3 mm and most preferred smaller than 0.1 mm.

Preferably the zones with different orientation of the anisotropy axis are randomly distributed.

The minimum size of an area of a region with a defined ratio of the zones oriented in the different directions depends on the optical detection system. Preferably, an average diagonal or diameter of a region is larger than 0.5 mm, more preferred larger than 1 mm and most preferred larger than 2 mm.

The number of different directions in a pattern of an element according to the invention is 2 or higher. In principle an element may have any number of directions. However, the measurement system and the evaluation algorithm may have limitations in resolution. Therefore, it may be reasonable to use less than 10 different orientation directions. Most preferred are pattern with 3, 4 and 5 different orientation directions.

The number of different information that can be encoded in a pattern depends on the number of different directions and the number of intensity levels that can be distinguished by the measurement and evaluation system. As an example, if a pattern has 3 different directions and the evaluation system allows to distinguish 20 different intensity levels associated with each of the directions than 8000 different information can be encoded in a single pattern.

Preferably, the anisotropic surface relief microstructure in the pattern encoding information by the orientation distribution is non-periodic.

Preferably, there are areas wherein the average structure depth of the non-periodic, anisotropic surface relief microstructure is larger than 60 nm, more preferably, the average structure depth of the microstructure is larger than 90 nm. For the generation of colors, the average structure depth of the microstructure is preferably larger than 180 nm, more preferred larger than 300 nm and most preferred larger than 400 nm. Preferred ranges of the average structure depth for providing distinctive colors are 180 nm to 230 nm, 240 nm to 280 nm, 290 nm to 345 nm, 365 nm to 380 nm and 430 nm to 600 nm.

Contrary to a periodic structure, which repeats itself after a certain interval, and which is therefore predictable once the structure of a period is known, as the surface profile of a non-periodic structure cannot be predicted at a distance from a known part of the structure. For determination of a surface profile being non-periodic the autocorrelation function and a related autocorrelation length can be used. The autocorrelation function of a surface profile can be understood as a measure for the predictability of the surface profile for two spatially separated points by a distance x in the plane.

The autocorrelation function AC(x) of a function P(x), such as the surface relief microstructure profile, is defined as
AC(x)=∫P(x′)·P(x′+x)·dx′

For a non-periodic or non-deterministic surface profile, the autocorrelation function decays rapidly with increasing x. On the other hand, for a deterministic surface profile found for instance in a grating, the autocorrelation function is modulated with a periodic function but the amplitude does not decay.

With the help of the autocorrelation function, a single characteristic number, an autocorrelation length L, can be defined. It is the length for which the envelope of the autocorrelation function decays to a certain threshold value. For the present purpose, a threshold value of 10% of AC(x=0) proved to be suitable.

In the context of the present invention it is preferred that a non-periodic, anisotropic surface relief microstructure has at least in one direction an averaged one-dimensional autocorrelation function AC(x) that has an envelope, which decays to 10% of the AC at x=0 within an autocorrelation length, wherein the autocorrelation length is smaller than three times an average lateral distance between adjacent transitions of top and bottom regions, such as hills and valleys. Preferably, the one direction is perpendicular to the anisotropy direction. Preferably, the anisotropic surface relief microstructure is also modulated along the anisotropy direction y, such that the envelope of an averaged autocorrelation function AC(y) decays to 10% of the AC at y=0 within an autocorrelation length, wherein the autocorrelation length is smaller than three times an average lateral distance between adjacent transitions of top and bottom regions along the anisotropy direction.

More preferred are surface relief microstructures, wherein the autocorrelation length is smaller than two times an average lateral distance between adjacent transitions of top and bottom regions. Even more preferred are surface relief microstructures, wherein the autocorrelation length is smaller than one average lateral distance between adjacent transitions of top and bottom regions.

Preferably, the autocorrelation length (L) is greater than one hundredth average lateral distance between adjacent transitions of top and bottom regions.

There are different known methods, which can be used to generate non-periodic, anisotropic surface relief microstructures, such as self-organization in copolymer or dewetting, laser ablation, electron- or ion beam lithography and nanoimprint lithography. The microstructures can, for example, simply be replicated by embossing using an embossing tool containing the microstructure.

A preferred method of manufacturing non-periodic, anisotropic surface relief microstructures is described in the international patent application WO-01/29148, the content of which is incorporated herein by reference. The method makes use of the so called monomer corrugation (MC) technology. It relies on the fact that phase separation of special mixtures or blends applied to a substrate is induced by crosslinking, for instance by exposure to ultraviolet radiation. The subsequent removal of non-crosslinked components leaves a structure with a specific surface topography. The term MC-layer is used for layers prepared according to this technology. Anisotropy of the microstructure can, for example, be achieved if liquid crystalline mixtures are used, which are aligned by an underlying alignment layer. By using an alignment layer with an orientation pattern, it is possible to create a patterned, non-periodic, anisotropic surface relief microstructures.

In preferred embodiments an element according to the invention generates colors when illuminated with white light. Manufacturing of suitable surface relief microstructures is disclosed in WO2007/131375, the content of which is incorporated herein by reference.

Preferably, the method of manufacturing a non-periodic, anisotropic surface relief microstructures according to the invention comprises the steps of coating a thin photo-alignment film on a substrate, generation of an orientation pattern by exposing individual areas of the photo-alignment film to linearly polarized UV light of different polarization directions, coating a blend of crosslinkable and non-crosslinkable liquid crystal materials on top of the photo-alignment film, cross-linking the liquid crystalline blend and removing the non-cross-linked material, for example using an adequate solvent.

Cross-linking of the liquid crystalline blend is preferable done by exposure to actinic light. The cross-linking process induces a phase separation and cross-linking of the liquid crystal prepolymer. The basic principles and the optical behavior of micro-corrugated thin-films are for example disclosed in the international patent application WO01/29148.

Preferably, optical elements according to the invention are at least partially reflective. The optical elements according to the invention therefore preferably comprise reflective or partially reflective layers using materials such as gold, silver, copper, aluminum, chromium or pigments. The reflective or partially reflective layers may further be structured such that they cover only part of the optical element. This can be achieved, for example, by structured deposition of the layer or by local de-metallization.

Reflection can also be caused by a transition to a material having a different index of refraction. Therefore, in a preferred embodiment of the invention the surface of the microstructure of an optical element according to the invention is at least partially covered with a dielectric material. Examples of high index refraction materials are ZnS, ZnSe, ITO or TiO2. Composite materials including nanoparticles of high index refraction materials could also be suitable. The cover medium may also be absorptive for certain colors to change the color appearance of the device.

Optionally, the surface relief microstructures of an optical element according to the invention may be sealed in order to protect the element against mechanical impact, contamination and in order to prevent unauthorized and illegal making of replicas of such elements. Therefore, optical elements according to the invention preferably comprise a sealing layer on top of the microstructure.

Optical elements produced according to the present invention can be used in different applications which deal with spatial modulation of the light intensity. Preferably the optical elements according to the invention are used as security elements in security devices. Specifically such security devices are applied to or are incorporated into documents, passports, licenses, stocks and bonds, coupons, cheques, certificates, credit cards, banknotes, tickets etc. against counterfeiting and falsification. The security devices further can also be applied as or incorporated into brand or product protection devices, or into means for packaging, like wrapping paper, packaging boxes, envelopes etc. Advantageously, the security device may take the form of a tag, security strip, label, fiber, thread, laminate or patch etc.

EXAMPLES

Sample Preparation

For the examples below the samples are made by first preparing a photo-alignment layer on a silicon wafer by spin-coating. An orientation pattern is created in the photo-alignment layer by sequential exposure of the different zones to linearly polarized uv-light of the desired polarization directions, which is provided by a laser scanning exposure system. The laser scanning system is controlled by a computer which provides the pattern information of the zones of different orientation direction. This avoids the use of multiple photo-masks and therefore allows fast generation of arbitrary orientation patterns. The pattern comprising randomly distributed zones corresponding to the different orientation directions with a desired area fraction is generated by a computer algorithm.

A patterned anisotropic surface relief microstructure is then manufactured using the MC-technology procedure disclosed in WO01/29148, which is incorporated herein by reference.

FIG.4shows a photograph of a silicon wafer with four different regions, each of them comprising a patterned anisotropic surface relief microstructure with information encoded in the pattern according to the invention.

The scattering characteristics of the samples of the examples below are measured by conoscopy using an ELDIM EZContrast 160R measurement system. The samples are illuminated with collimated white light from a normal incidence angle with regard to the surface plane of the sample and the luminosity of the reflected light is measured as a function of the azimuthal (0°-360°) and zenithal (0°-80°) angle.

FIG.5illustrates the measurement geometry, wherein the azimuthal angle is referred to as θ and the polar angle is referred to as (p.

The evaluation can either take into account the full distribution of the scattered light or it may use only part of the spatial light distribution if this part is characteristic and provides sufficient data to evaluate the encoded information. The evaluation of the samples of the examples below has been made by only taking into account the data measured for a constant polar angle φ of 30° as indicated inFIG.5by the numeral25.

Example 1

An optical element according to the invention is made as described above. The element comprises a patterned anisotropic surface relief microstructure with two types of regularly distributed zones, which differ in the anisotropy direction. The different zones are arranged in checkerboard pattern, wherein the zones corresponding to the different directions are represented as black and white, respectively, as shown inFIG.6a. Each of the zones is 50 μm×50 μm. The orientation direction in the first type of zone is oriented at 0° with regard to a reference direction and the first zones have a fraction f1=50%. The orientation direction in the second type of zone is oriented at 90° with regard to a reference direction and the zones have a fraction f2=50%.

FIG.6bshows the result of the conoscopic measurement. For determination the fraction of the different zones a curve is created using only the data for the polar angle 30°, as described above. For the fitting algorithm it is assumed that the scattering profiles related to the different types of zones have a Gaussian distribution. The result from the evaluation of the sample is that the fraction f1of zones 1 is 48.5% and fraction f2of zones 2 is 51.5%. The result is very close to the fraction that has been defined by the distribution pattern. Hence the information encoded in the pattern is decoded.

Example 2

An optical element according to the invention is made as described above. The element comprises a patterned anisotropic surface relief microstructure with three types of randomly distributed zones, which differ in the anisotropy direction.FIG.7ashows the pattern that has been created, wherein the zones corresponding to the different directions are represented in three different grey levels. The size of the smallest zone (unit zone) is 50 μm×50 μm, but because of the random arrangement larger zones are formed as well. The unit zone has the form of a square. The orientation direction in the first type of zone is oriented at 0° with regard to a reference direction and the first zones have a fraction f1=50%. The orientation direction in the second type of zone is oriented at 45° with regard to a reference direction and the zones have a fraction f2=30%. The orientation direction in the third type of zone is oriented at 90° with regard to a reference direction and the zones have a fraction f3=20%.

FIG.7bshows the result of the conoscopic measurement. The evaluation is made as described in example 1. The result from the evaluation of the sample is that the fraction f1of zones 1 is 52.5%, fraction f2of zones 2 is 33.4% and fraction f3of zones 3 is 14.1%. The result is very close to the fraction that has been defined by the distribution pattern. Hence the information encoded in the pattern is decoded.

Example 3

An optical element according to the invention is made as described above. The element comprises a patterned anisotropic surface relief microstructure with three types of randomly distributed zones, which differ in the anisotropy direction.FIG.8ashows the pattern that has been created, wherein the zones corresponding to the different directions are represented in three different grey levels. The size of the smallest zone (unit zone) is 50 μm×50 μm, but because of the random arrangement larger zones are formed as well. The unit zone has the form of a square. The orientation direction in the first type of zone is oriented at 0° with regard to a reference direction and the first zones have a fraction f1=33.3%. The orientation direction in the second type of zone is oriented at 45° with regard to a reference direction and the zones have a fraction f2=33.3%. The orientation direction in the third type of zone is oriented at 90° with regard to a reference direction and the zones have a fraction f3=33.3%.

FIG.8bshows the result of the conoscopic measurement. The evaluation is made as described in example 1. The result from the evaluation of the sample is that the fraction f1of zones 1 is 34.1%, fraction f2of zones 2 is 35.0% and fraction f3of zones 3 is 30.9%. The result is very close to the fraction that has been defined by the distribution pattern. Hence the information encoded in the pattern is decoded.

Example 4

An optical element according to the invention is made as described above. The element comprises a patterned anisotropic surface relief microstructure with three types of randomly distributed zones, which differ in the anisotropy direction.FIG.9ashows the pattern that has been created, wherein the zones corresponding to the different directions are represented in three different grey levels. The unit zone has the form of a triangle. The orientation direction in the first type of zone is oriented at 0° with regard to a reference direction and the first zones have a fraction f1=33.3%. The orientation direction in the second type of zone is oriented at 60° with regard to a reference direction and the zones have a fraction f2=33.3%. The orientation direction in the third type of zone is oriented at 120° with regard to a reference direction and the zones have a fraction f3=33.3%.

FIG.9bshows the result of the conoscopic measurement. The evaluation is made as described in example 1. The result from the evaluation of the sample is that the fraction f1of zones 1 is 33.2%, fraction f2of zones 2 is 33.1% and fraction f3of zones 3 is 33.7%. The result is very close to the fraction that has been defined by the distribution pattern. Hence the information encoded in the pattern is decoded.