Optically readable physical unclonable functions on inorganic materials

Abstract

According to a first aspect of the present invention, there is provided a method of making an optically readable PUF on a surface of a first material, wherein the first material comprises an inorganic material, the method comprising treating the surface so as to create a plurality of colour centres thereon.

Description

[0001] Optically readable physical unclonable functions on inorganic materials

[0002] The present invention relates generally to methods of producing optically readable physical unclonable functions (PUFs) and articles comprising optically readable PUFs.

[0003] There is often a need to prove, or disprove, the authenticity of an object or similar. For instance, this might be needed for security purposes, for example to allow or prevent access to certain functionality associated with the object, or simply to allow a user or consumer of the object to be satisfied that they are using an authentic object. It will be appreciated that such tests for authenticity find use in the fields of anti-counterfeiting, security and so on.

[0004] In order to be able to prove that an object is an authentic object, or in other words to authenticate an object, that object might be provided with a unique identifier in one form or another. “Unique” might not necessarily mean that it is impossible for another object to have the same identifier, but instead that it is statistically highly unlikely for this to be the case, or in other words for the identifier to be accidentally stumbled across by guesswork or simple trial and error. The very same “uniqueness” might be used in other ways, too, for example for highly targeted marketing or data acquisition with respect to the object or a user or consumer of that object.

[0005] A unique identifier might, for example, take the form of or be derived from a physical (sometimes referred to as physically) unclonable function (a PUF). This might be in the form of a device or other element, the properties of which depend on small variations in construction or fabrication or similar, but which nevertheless can be used to provide a unique identifier. For instance, in a vast array of memory cells, a certain number of memory cells may be defective, and this number or arrangement of defective cells will be different for different arrays that are produced. Thus, this is a simple example of a unique identifier. Another example might be, for instance, a capacitance or resistance of an electrical component, based on the thickness of layers within that component, or the extent of those layers, and so on. Due to tolerances in manufacturing, each component will likely have a slightly different construction, and so a slightly different, and unique, electrical property.

[0006] Unique identifiers do not necessarily need to be based on electrical principles. For instance, physical unclonable functions may be probed or otherwise challenged optically in order to determine a unique identifier. For instance, the way in which one or more optical emitters are provided on an object may, as above, yield an overall emission spectrum or map which is unique, again providing a readable unique identifier.

[0007] Traditionally, the generation of unique identifiers, and / or associated use of physical unclonable functions, have been based on macroscopic effects. More recently though, it has been proposed to incorporate quantum mechanical effects in the generation of unique identifiers. In these more recent examples, for instance, an electrical component exhibiting quantum mechanical confinement (e.g. a resonant tunnelling diode) may be used as a quantum mechanical based physical unclonable function. The electrical properties of such a device or structure, and thus the unique identifier, are based on quantum mechanical principles. Similarly, optical based physical unclonable functions may be based on the emissions spectra of quantum dots, or 2-D materials, or similar, located on an object. In both cases, it may be extremely difficult, if not impossible, to be able to physically copy a security element (e.g. being or comprising a physical unclonable function) based on quantum mechanical effects. This is to the extent that the unique identifier provided by such an element may not be circumvented, and certainly not in any practical timeframe.

[0008] It is an example aim or example embodiments of the present invention to at least partially overcome or avoid one or more disadvantages of the prior art, whether identified herein or elsewhere, or to at least provide a viable alternative.

[0009] According to the present invention there are provided products and methods as set forth in the claims that follow. Other features of the invention will be apparent from the dependent claims, and the description which follows.

[0010] According to a first aspect of the present invention, there is provided a method of making an optically readable PUF on a surface of a first material, wherein the first material comprises an inorganic material, the method comprising treating the surface so as to create a plurality of colour centres thereon.

[0011] The optically readable PUF (physical unclonable function) can be used to provide a unique identifier which can be verified by optical means. The properties of the optically readable PUF may depend on small variations in construction or fabrication or similar.

[0012] Inorganic materials such as metals may be robust and have properties such as chemical inertness or resistance to mechanical wearing which enable their use in harsh environments, such as in moist environments, on exposure to acids or (corrosive) gases, or at extreme temperatures or pressures. Adding an optically readable PUF to such materials via a sticker or label, for example, faces the problem that the optically readable PUF may degrade at a faster rate than the inorganic material. Therefore, the optically readable PUF may require replacement once or multiple times during the lifetime of the substrate. The method of the first aspect advantageously allows optically readable PUFs to be created on the surface of inorganic materials via treatment of the inorganic material, such that the resulting optically readable PUF has a level of robustness similar to that of the substrate material. Therefore, the optically readable PUF may advantageously require little or no replacement during the lifetime of the substrate.

[0013] The invention may be associated with further advantages. The method of the first aspect may directly integrate the optically readable PUF in the surface of the inorganic material such that there is no increase in thickness of the inorganic material. This may be important for objects which require precise dimensions, such as interconnected moving parts. Furthermore, it may allow the optically readable PUF to be more aesthetically pleasing and may improve the security of the optically readable PUF by making it covert i.e. the user may not be aware of the optically readable PUF unless instructed to read it.

[0014] The optically readable PUF may be used as a security element. The optically readable PUF may include an alignment mark, such as a geometric shape or a logo, to allow an optical reading device to define the area to be read. The alignment mark may also allow an optical reading device to use that mark to correct for scale, perspective and any transforms (e.g. mirror image) that may be applied as part of the reading of the PUF. The alignment mark is preferably visible to the naked eye. This allows the user to easily point the optical reading device at the general location of the optically readable PUF on the surface of the inorganic material.

[0015] A ‘unique’ identity, e.g. serial number or QR code, may be formed adjacent to or overlapping with the optically readable PUF to enable further authentication of the PUF. The unique identity is preferably visible to the naked eye. The unique identity may act as an alignment mark. Therefore, it may not be necessary to include an (e.g. separate or dedicated) alignment mark in the optically readable PUF if a unique identify is formed.

[0016] The unique identity may be formed with the same treatment step used to form the optically readable PUF. Alternatively, the unique identity is formed in a different step to the optically readable PUF, and suitably from a different material to the optically readable PUF. The optically readable PUF and the unique identity are preferably formed so as to overlap. In such embodiments, the unique identity may comprise the optically readable PUF, the optically readable PUF may comprise the unique identity, or the optically readable PUF itself may be formed into a unique identity. For example, the optically readable PUF may be in the shape of a QR code or may comprise a QR code as a portion thereof.

[0017] The first material comprises an inorganic material. The first material may consist essentially of the inorganic material. Preferably, the first material consists of the inorganic material. The inorganic material is suitably a crystalline material. The inorganic material may be a metallic material or a ceramic material. Preferably, the inorganic material is a metallic material. The metallic material may comprise aluminium, iron, titanium, nickel, cobalt or an alloy thereof. The alloy may be a superalloy. Alloys typically contain aluminium, titanium, chromium, and / or a rare earth elements such as yttrium. Superalloys typically contain tungsten, molybdenum, niobium, tantalum, a ceramic and / or a ceramic-metal mix.

[0018] The metallic material may comprise aluminium. The metallic material may comprise iron or an alloy thereof, such as steel, austenite, or Incoloy. The metallic material may comprise titanium or an alloy thereof. The metallic material may comprise nickel or an alloy thereof, preferably a superalloy thereof such as Hastelloy, Waspaloy, or Rene 41 . The metallic material may comprise cobalt or an alloy thereof, preferably a superalloy such as Inconel.

[0019] In some embodiments, the inorganic material is a ceramic material. The ceramic material may comprise an oxide compound, carbide compound or nitride compound. For example, the ceramic material may comprise silicon dioxide, silicon carbide, or boron nitride.

[0020] Forming a plurality of colour centres on the surface of the first material results in an optically readable PUF. Suitably, the plurality of colour centres is randomly distributed. By “colour centres” is meant crystal defects on the surface which are optically distinguishable from other areas of the surface. The colour centres may be absorbers and / or emitters of electromagnetic radiation. Suitably, the colour centres are emitters of electromagnetic radiation, or in otherwords generally able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. Preferably, the colour centres are fluorescent.

[0021] Examples of crystal defects include substitute atoms, vacancies, point defects, line defects, interstitial defects, screw defects, and combinations thereof. Preferably, the defects comprise substitute atoms. By selecting particular substitute atoms the absorption and / or emission wavelength of the colour centres may readily be controlled.

[0022] The optically readable PUF suitably comprises from 1 to 100 colour centres per 10 microns square. Such a relatively low concentration of colour centres may advantageously facilitate the reading of the colour centres by an optical reading device, particularly a reading device without advanced optical elements such as a microscope. An excessive concentration of colour centres may prevent a reading device from distinguishing between individual colour centres. The PUF is suitably readable using a handheld optical reading device, such as a mobile telephone (e.g. a smartphone) or a tablet.

[0023] The method of the first aspect may further comprise the step of forming a first layer on the surface of the first material. This step may comprise reacting the surface of the first material and / or depositing a coating on the surface of the first material. Suitably, the first material remains present as a substrate (or base layer) for the first layer. Preferably, the “surface of the first material” refers to the surface of the first material which is exposed to the atmosphere, i.e. the surface which is available for treatment or coating.

[0024] The first layer suitably comprises the plurality of colour centres. Preferably, the first layer comprises the optically readable PUF. The step of treating the surface of the first material so as to create a plurality of colour centres thereon and the step of forming a first layer on the surface of the first material are suitably carried out simultaneously. The step of treating the surface of the first material so as to create a plurality of colour centres thereon may comprise the step of forming a first layer on the surface of the first material.

[0025] The first layer suitably forms a protective layer on the surface of the first material. The first layer is suitably less susceptible to physical or chemical degradation than the first material. For example, the first layer may be stronger or harder than the first material, have a higher heat tolerance, or be more chemically inert, e.g. more resistant to rusting.

[0026] The first layer suitably comprises an inorganic material. The first layer may consist essentially of the inorganic material. Preferably, the first layer consists of the inorganic material. The inorganic material is suitably a crystalline material. When the first layer comprises a crystalline material, the first material may be a non-crystalline material. The inorganic material may be a metallic material or a ceramic material. When the first material comprises a metallic material, the first layer may be a compound of the metallic material. The first layer may be a conductor, a semiconductor, or an insulator. Suitably, the first layer is a semiconductor, such as a wide bandgap semiconductor. The first layer may be transparent. A transparent first layer may advantageously facilitate the reading of the optically readable PUF.

[0027] The first layer may be formed on a portion of the surface of the first material. In such embodiments, the first layer may be formed as a shape for or design on the surface of the first material. For example, the first layer may form a unique identity as described herein, such as a serial number or QR code.

[0028] Preferably, the first layer is formed on the entire surface of the first material. A first layer covering the entire surface of the first material suitably provides better protection than a first layer covering only a portion of the surface of the first material.

[0029] Reacting the surface of the first material to form the first layer may comprise exposing the surface of the first material to photons or charged particles, such as plasma, a laser beam, an electron beam, or an ion beam. Preferably, the surface of the first material is exposed to a laser beam, electron beam, or an ion beam. Exposing the surface of the first material to photons or charged particles may comprise etching a portion of the surface, such as etching a design onto the surface.

[0030] Reacting the surface of the first material to form the first layer may comprise passivating the surface of the first material. In such embodiments, the first material is preferably a metallic material. The passivation may be carried out by anodising the first material. The passivation suitably comprises exposing the surface of the first material to a passivating agent. The passivating agent suitably reacts with the first material to form the first layer (which may be referred to as a passivation layer). The passivating layer typically comprises the first material and the passivating agent. The passivating agent suitably comprises a non-metal. In some embodiments, the passivation layer comprises a metal and a non-metal. The non-metal is suitably selected from oxygen, carbon, or nitrogen. The non-metal preferably comprises oxygen, which may be in the form of gaseous oxygen. In some embodiments, the first material is a metallic material and is passivated in the presence of oxygen to form a first layer comprising a metal oxide. The passivation may be used for forming the first layer on the entire surface of the first material. However, the passivation may also be used to form the first layer on a portion of the surface of the first material, for example by using a mask or template on the surface during the passivation.

[0031] The step of forming the first layer may comprise depositing a coating on the surface of the surface of the first material. The coating may be deposited by any suitable method. Suitable methods include chemical vapour deposition (CVD), atomic layer deposition (ALD), plasma treatment, annealing, anodisation, galvanising, thermal spray, electrospray, electroplating, and sherardising. The coating suitably comprises a ceramic material. When the coating comprises a ceramic material, the first material is suitably a metallic material. The ceramic material may comprise an oxide, carbide or nitride. For example, the ceramic material may comprise silicon dioxide, silicon carbide, or boron nitride.

[0032] The first layer is suitably formed under conditions which result in the formation of colour centres in the first layer. In some embodiments, the first layer is formed at a temperature selected so as to favour the formation of colour centres such as crystal defects. Typically, a rapid decrease in temperature during the formation of the first layer causes the formation of smaller crystal domains. Since crystal defects are often found at the boundaries of crystal domains, the presence of smaller crystal domains will typically result in a larger number of crystal defects.

[0033] In some preferred embodiments, the first layer is formed in the presence of a dopant, and / or the first layer is treated with a dopant following the formation of the first layer. The dopant suitably results in the formation of colour centres. Suitably, the colour centres are randomly distributed in the first layer. Preferably, the colour centres are substitute atoms which are suitably provided by the dopant. Substitute atoms in crystalline materials may advantageously result in strongly emitting colour centres which can be tuned according to the type of substitute atom present.

[0034] The first layer may be formed in the presence of a dopant. The dopant is suitably comprised in a gas or plasma, preferably a gas. In some preferred embodiments, the first layer is formed by reacting the surface of the first material in the presence of a gas or plasma comprising a dopant, preferably by exposing the surface of the first material to photons or charged particles. The gas or plasma suitably comprises the dopant in a concentration of less than 100 ppm, such as less than 10 ppm, for example less than 1 ppm. Controlling the concentration of the dopant in the gas or plasma, for example by avoiding the use of high concentrations, may advantageously facilitate control of the formation of colour centres.

[0035] In some embodiments, the dopant is a solid or liquid. The solid or liquid dopant may be deposited on the surface of the first material prior to reacting the surface to form the first layer.

[0036] The first layer may be treated with a dopant following the formation of the first layer. For example, a dopant may be introduced into the first layer by an ion-implantation process, such as by irradiating the first layer with ions. In some embodiments, the dopant is a solid or liquid and is deposited on the surface of the first layer, followed by heating (e.g. annealing) the first layer to form colour centres in the first layer. The dopant may be deposited on a portion of the surface of the first layer. This suitably favours the formation of colour centres in a selected portion of the first layer, and may be desirable for cost or aesthetic reasons, for example.

[0037] The dopant suitably comprises a metal. The metal may comprise barium, calcium, magnesium, chromium, titanium, vanadium, manganese, iron, cobalt, nickel, copper, molybdenum, and / or erbium. The dopant may comprise an organic component. For example, the dopant may comprise an organic moiety bound to the metal. The dopant may comprise an organometallic compound.

[0038] The method of the first aspect may further comprise the step of forming a second layer on the surface of the first layer. This step may comprise reacting the surface of the first layer and / or depositing a coating on the surface of the first layer, preferably by depositing a coating on the surface of the first layer. Suitable methods of reacting the surface and depositing a coating on the surface are as described in relation to the formation of the first layer. For example, the step of forming the second layer may comprise depositing a coating on the surface of the first layer by chemical vapour deposition (CVD), atomic layer deposition (ALD), plasma treatment, annealing, anodisation, galvanising, thermal spray, electrospray, electroplating, and sherardising. Forming the second layer by deposition of a coating has the advantage that it minimises disruption to the first layer.

[0039] Suitably, the second layer is formed on the entire surface of the first layer. In embodiments where the first layer is formed on only a portion of the surface of the first material, the second layer may be formed on at least a portion of the remaining surface of the first material. Preferably, the second layer is formed on the entire surface of the first layer and the entire remaining surface of the first material such that the entire surface of the first material is covered.

[0040] The second layer suitably comprises an inorganic material. The second layer may consist essentially of the inorganic material. Preferably, the second layer consists of the inorganic material. The inorganic material is suitably a crystalline material. The inorganic material may be a metallic material or a ceramic material. The second layer may be a conductor, a semiconductor, or an insulator. Suitably, the second layer is a semiconductor, such as a wide band-gap semiconductor. The second layer may be transparent. A transparent second layer may advantageously facilitate the reading of the optically readable PUF. Suitably, the second layer is free or substantially free from the dopant.

[0041] The second layer may advantageously protect the first layer and increase the durability of the colour centres in the first layer. The second layer may have the same chemical composition as the first layer, but suitably comprises fewer colour centres than the first layer. The second layer may be formed at a different temperature to the first layer such that the second layer comprises fewer colour centres than the first layer. For example, the first layer and the second layer may both be formed at an elevated temperature (e.g. a temperature greater than 100°C) followed by cooling to room temperature (e.g. about 20°C), but the rate of cooling of the second layer may be less than the rate of cooling of the first layer. This suitably results in a second layer having larger crystal domains than the first layer. Therefore, the second layer may have fewer crystal defects than the first layer. Suitably, the second layer has better mechanical properties than the first layer, such as a higher melting temperature or better wear resistance.

[0042] The first layer may have a thickness of at least 0.5 nm, such as at least 5 nm, preferably at least 50 nm, for example at least 100 nm. The first layer may have a thickness of from 0.5 to 5000 nm, such as from 5 to 2500 nm, preferably from 50 to 1000 nm, for example from 100 to 500 nm. The second layer may have a thickness of at least at least 50 nm, preferably at least 100 nm. The second layer may have a thickness of from 50 to 5000 nm, preferably from 100 to 2500 nm. Where the first layer is exposed (and no second layer is present), the thickness of the first layer is suitably at least 50 nm. This advantageously reduces the depletion of colour centres in the first layer due to surface effects or chemical or mechanical degradation. The presence of a second layer has a similar effect, and thus a second layer may advantageously be present when the first layer has a thickness of less than 50 nm.

[0043] In some embodiments, the first material is aluminium. In such embodiments, the method of the first aspect suitably comprises reacting the surface of the aluminium in the presence of oxygen to form a first layer comprising aluminium oxide, preferably AI2O3. The aluminium oxide is suitably in the form of sapphire. Sapphire is advantageously a strong, hard, wear-resistant and transparent wide band-gap semiconductor or insulator. Reacting the surface of the aluminium may comprise anodising the surface of the aluminium or exposing the surface of the aluminium to photons or charged particles, such as plasma, a laser beam, an electron beam, or an ion beam. Preferably, the surface of the aluminium is reacted in the presence of a dopant. The dopant suitably comprises a metal. The metal suitably comprises chromium, titanium, vanadium, manganese, iron, cobalt, nickel, and / or copper. Metal-based colour centres in sapphire may advantageously be strong emitters of visible light. For example, chromium-based colour centres in sapphire are known to be emitters of red light.

[0044] In some embodiments, the first material is iron or an alloy thereof, such as steel. In such embodiments, the method of the first aspect suitably comprises reacting the surface of the iron or alloy thereof to form a first layer. The surface of the iron or alloy thereof may be reacted in the presence of silicon and oxygen to form a silicate material. Silicate materials may advantageously be chemically robust transparent semiconductors or insulators. The silicate material may be a garnet, such as an iron aluminium garnet (FesA^SisO^). The garnet may comprise colour centres having different colours due to variations in the oxidation state of individual iron atoms, or substitutions of the iron atoms for other metals such as calcium or magnesium. The method of the first aspect may comprise reacting the surface of the iron or alloy thereof in the presence of silicon, oxygen, aluminium, and a dopant to form a garnet layer, suitably wherein the dopant comprises a metal such as calcium or magnesium.

[0045] In some embodiments, the first material is titanium or an alloy thereof. In such embodiments, the method of the first aspect suitably comprises reacting the surface of the titanium or alloy thereof to form a first layer. The first layer is suitably a crystalline material. The surface of the titanium or alloy thereof may be reacted in the presence of oxygen to form an oxide material, or in the presence of silicon and oxygen to form a silicate material. Suitably, the surface of the titanium or alloy thereof is reacted in the presence of silicon, oxygen, and barium to form benitoite (BaTiSisOg). Alternatively, the surface of the titanium or alloy thereof is reacted in the presence of silicon, oxygen, and calcium to form titanite (CaTiSiOs). The benitoite or titanite suitably comprise a random distribution of colour centres. In some embodiments, the method of the first aspect comprises depositing a layer of silicon dioxide on the surface of the first material. Silicon dioxide may advantageously be transparent, strong and inert. The silicon dioxide may comprise colour centres due to the presence of crystal defects. The colour centres in the silicon dioxide may comprise heavy ions such as argon ions. The method may comprise irradiating the silicon dioxide with heavy ions such as argon ions to form colour centres in the silicon dioxide.

[0046] In some embodiments, the method of the first aspect comprises depositing a layer of silicon carbide on the surface of the first material. The silicon carbide may comprise colour centres due to the presence of crystal defects. The colour centres may comprise a metal dopant such as titanium, chromium, vanadium, molybdenum, and / or erbium. The dopant may be present during the deposition of the silicon carbide layer. Alternatively, the silicon carbide layer may be treated with the dopant after formation of the silicon carbide layer, for example by an ion-implantation process.

[0047] In some embodiments, the method of the first aspect comprises depositing a layer of boron nitride on the surface of the first material. Boron nitride is advantageously a thermally and chemically resistant refractory compound. The boron nitride is suitably a transparent wide bandgap semiconductor. The boron nitride suitably forms a protective layer on the surface of the first material. Hexagonal boron nitride is preferred. The boron nitride may be deposited at a thickness of from 0.5 to 50 nm, such as from 0.5 to 10 nm. The boron nitride may comprise colour centres due to the presence of crystal defects.

[0048] The method of the first aspect may comprise an additive manufacturing process. Additive manufacturing processes may sometimes be known as 3D printing and typically involve the deposition and solidification of layers of material. Suitably, the additive manufacturing method comprises laser sintering a precursor material (such as a powder) so as to form a solid layer. The solid layer may correspond to the first material, first layer, or second layer as described herein. Suitably, the laser sintering creates a plurality of colour centres on the surface of the solid layer. The laser sintering may be carried out in the presence of a dopant as described herein.

[0049] According to a second aspect of the present invention, there is provided an article formed from a first material, wherein the first material is an inorganic material, and the surface of the first material comprises an optically readable PUF comprising a plurality of colour centres.

[0050] Suitable features of the first material, the optically readable PUF and the colour centres may be as described in relation to the first aspect. Preferably, the optically readable PUF is made according to the method of the first aspect.

[0051] The article of the second aspect suitably comprises a first layer on the surface of the first material. The first layer is suitably as defined in relation to the first aspect.

[0052] The first layer may be a passivation layer. In such embodiments, the first material is preferably a metallic material. The passivation layer is suitably formed by passivating the surface of the first material. The passivation layer suitably comprises the first material and a passivating agent. The passivating agent suitably comprises a non-metal. In some embodiments, the passivation layer comprises a metal and a non-metal. The non-metal is suitably selected from oxygen, carbon, or nitrogen. The non-metal preferably comprises oxygen, which may be in the form of gaseous oxygen. In some embodiments, the first layer is a passivation layer comprising a metal oxide.

[0053] The first layer suitably comprises a dopant. The dopant is suitably as defined in relation to the first aspect.

[0054] The article of the second aspect suitably comprises a second layer on the surface of the first layer. The second layer is suitably as defined in relation to the first aspect.

[0055] The article of the second aspect suitably comprises an alignment mark. The alignment mark may allow an optical reading device to define the area to be read and / or to allow the optical reading device to use that mark to correct for scale, perspective and any transforms (e.g. mirror image) that may be applied as part of the reading of the PUF.

[0056] The article of the second aspect may be a component suitable for use in industry. The article may be a component suitable for use in the construction, marine, aviation, or automobile industry. For example, the article may be a component of a vehicle such as an aeroplane, ship or automobile.

[0057] According to a third aspect of the present invention, there is provided the use of a plurality of colour centres on the surface of a first material as an optically readable PUF, wherein the first material is an inorganic material.

[0058] Suitable features of the first material, the optically readable PUF and the colour centres may be as described in relation to the first aspect or the second aspect. The advantages and benefits will be the same or similar. Preferably, the colour centres or optically readable PUF are made according to the method of the first aspect.

[0059] The plurality of colour centres are suitably comprised in a first layer on the surface of the first material. The first layer is suitably as defined in relation to the first aspect or the second aspect.

[0060] Alternatively, the colour centres may be pre-existing in the first material. The inventors have found that some existing materials comprise colour centres which have been created unintentionally, and which have not previously been used as an optically readable PUF. The use of the third aspect may therefore advantageously ‘retrofit’ an optically readable PUF onto an existing material.

[0061] Suitably, the optically readable PUF is used as a security element. The use may comprise reading the PUF using an optical reading device, for example to verify the authenticity of an article comprising the first material.

[0062] The use may comprise defining an area of the optically readable PUF to be read in relation to an alignment mark on the surface of the first material.

[0063] Brief Description of the Drawings

[0064] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, byway of example, to the accompanying Figures in which:

[0065] Figure 1 depicts a perspective view of an optically readable PUF being formed on the surface of an article according to the method of the first aspect of the present invention.

[0066] Figure 2 schematically depicts a cross-section of an article according to the second aspect of the present invention.

[0067] Detailed Description of the Example Embodiments

[0068] Figure 1 depicts a perspective view of an article 10 having a bulk material 11 formed of aluminium. A laser beam 12 is used in an atmosphere 13 containing oxygen and a trace amount of chromium to form a layer 14 of aluminium oxide (sapphire) on a portion of the surface of the bulk material 11 , said layer 14 comprising a random distribution of chromium dopant which forms red-emitting colour centres. The laser may also be used to etch a design 15, such as a QR code, onto the surface of the bulk material 11 . Figure 2 schematically depicts a cross-section of an article 20 having a bulk material 21 formed of aluminium. The article 20 comprises a first layer 22 of aluminium oxide (sapphire) formed on a portion of the surface of the bulk material 21 and comprising a random distribution of chromium dopant which forms red-emitting colour centres. The article also comprises a second layer 23 formed on the entire surface of the first layer 22 and the entire remaining surface of the bulk material 21 . The second layer 23 is free of chromium dopant.

[0069] Optically readable PUFs made on the surface of inorganic materials as described herein generally have a high level of durability, especially if they are formed in a crystalline material. This allows the optically readable PUF to be used for longer periods under the harsh conditions to which the inorganic materials may be exposed in use. For example, the optically readable PUF may be strong, wear-resistant, resistant to high temperatures, and resistant to corrosive chemical environments. The optically readable PUF may also form part of a layer which protects and increases the useful lifetime of the inorganic material.

[0070] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0071] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0072] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0073] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0074] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims1 . A method of making an optically readable PUF on a surface of a first material, wherein the first material comprises an inorganic material, the method comprising treating the surface so as to create a plurality of colour centres thereon.

2. The method of claim 1 , wherein the inorganic material is a crystalline material.

3. The method of claim 1 or claim 2, wherein the inorganic material is a metallic material or a ceramic material.

4. The method of claim 3, wherein the metallic material comprises aluminium, iron, titanium, nickel, cobalt or an alloy thereof.

5. The method of claim 3, wherein the ceramic material comprises an oxide compound, carbide compound, or nitride compound.

6. The method of any preceding claim, wherein the optically readable PUF comprises 1 to 100 colour centres per 10 microns square.

7. The method of any preceding claim, further comprising the step of forming a first layer on the surface of the first material, and, optionally, wherein the step of forming the first layer comprises reacting the surface of the first material and / or depositing a coating on the surface of the first material.

8. The method of claim 7, wherein reacting the surface of the first material comprises exposing the surface of the first material to photons or charged particles.

9. The method of claim 7, wherein reacting the surface of the first material comprises passivating the surface of the first material.

10. The method of any one of claims 7 to 9, wherein the first layer is formed in the presence of a dopant, and / or the first layer is treated with a dopant following the formation of the first layer.

11. The method of claim 10, wherein the dopant comprises barium, calcium, magnesium, chromium, titanium, vanadium, manganese, iron, cobalt, nickel, copper, molybdenum, and / or erbium.

12. The method of any one of claims 7 to 11 , further comprising the step of forming a second layer on the surface of the first layer.

13. An article formed from a first material, wherein the first material is an inorganic material, and the surface of the first material comprises an optically readable PUF comprising a plurality of colour centres.

14. The article of claim 13, wherein the optically readable PUF is made according to the method of any one of claims 1 to 12.

15. Use of a plurality of colour centres on the surface of a first material as an optically readablePUF, wherein the first material is an inorganic material.

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