WO2009112999A1 - Rf identification tag - Google Patents

Rf identification tag Download PDF

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Publication number
WO2009112999A1
WO2009112999A1 PCT/IB2009/050959 IB2009050959W WO2009112999A1 WO 2009112999 A1 WO2009112999 A1 WO 2009112999A1 IB 2009050959 W IB2009050959 W IB 2009050959W WO 2009112999 A1 WO2009112999 A1 WO 2009112999A1
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WO
WIPO (PCT)
Prior art keywords
tag
circuits
resonator circuits
capacitance
coil
Prior art date
Application number
PCT/IB2009/050959
Other languages
French (fr)
Inventor
Boris Skoric
Boudewijn R. De Jong
Antonius H. M. Blom
Geert J. Schrijen
Antonius J. M. Nellissen
Ronald J. Asjes
Matheus G. J. Bel
Original Assignee
Koninklijke Philips Electronics N.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2009112999A1 publication Critical patent/WO2009112999A1/en

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Classifications

    • GPHYSICS
    • G06COMPUTING; CALCULATING OR COUNTING
    • G06KGRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K19/00Record carriers for use with machines and with at least a part designed to carry digital markings
    • G06K19/06Record carriers for use with machines and with at least a part designed to carry digital markings characterised by the kind of the digital marking, e.g. shape, nature, code
    • G06K19/067Record carriers with conductive marks, printed circuits or semiconductor circuit elements, e.g. credit or identity cards also with resonating or responding marks without active components
    • G06K19/0672Record carriers with conductive marks, printed circuits or semiconductor circuit elements, e.g. credit or identity cards also with resonating or responding marks without active components with resonating marks
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/32Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials
    • H04L9/3271Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials using challenge-response
    • H04L9/3278Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials using challenge-response using physically unclonable functions [PUF]
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L2209/00Additional information or applications relating to cryptographic mechanisms or cryptographic arrangements for secret or secure communication H04L9/00
    • H04L2209/80Wireless
    • H04L2209/805Lightweight hardware, e.g. radio-frequency identification [RFID] or sensor

Definitions

  • This invention relates to radio frequency (RF) identification tag having a random resonant frequency that serves as a unique identifier.
  • Counterfeiting is a serious problem affecting many areas of industry.
  • the severity of the problem is reflected in the large number of anti-counterfeiting (AC) technologies available, including security inks, gratings, holograms, magnetic dots, fluorescent and phosphorescent materials, watermarks, rare chemicals, electromagnetic codes, etc.
  • AC anti-counterfeiting
  • Radio Frequency Identification is increasingly being employed as an AC measure.
  • RFID tags are currently too costly for item- level tagging.
  • a very promising alternative is the use of so-called chipless RFID, i.e. completely passive tags that do not contain an integrated circuit.
  • chipless RFID is a track-and-trace technology vulnerable to cloning of the tags.
  • the authenticity marker should be something that cannot be controlled by its maker (hereinafter referred to as "manufacturer- resistance").
  • the marker is a feature intrinsic to the product that has to be protected, but when this is not achievable, the product can be tagged with an unclonable marker.
  • a simple resonator circuit also known as a 'tank circuit' or LC-circuit
  • LC capacitor
  • the circuit When placed in an electromagnetic field whose magnetic part couples well into the coil, the circuit will absorb a frequency-dependent amount of power from the field, with a peak at ⁇ , and this can be measured by a measurement device.
  • the inductance of a coil is a complicated function of the coil geometry.
  • the capacitance of a parallel plate capacitor is dependent on the plate area, the relative dielectric constant of the material between the plates and the separation between the plates. It will thus be apparent that that the resonant frequency of the circuit can be randomized by making any of the parameters affecting capacitance or inductance random.
  • the unique identifier thus obtained for each tag can be stored in a secure database or certified by a Certification Authority. Items that do not carry a valid tag (i.e. listed in the database or a certificate) are considered counterfeits. In order to create a counterfeit that passes verification, an attacker must either make a precise physical clone of the random resonator or forge a digital signature, or modify the information in a secure database.
  • n tags do not offer significantly more protection than one. This problem can be mitigated to a certain extent by placing multiple resonators close together in the same mechanical constructions. However, the main drawback of this approach is that the resonance peaks become less distinct. Referring to Figure Ia of the drawings, the frequency response from two completely separate LC circuits has the form of two distinct resonance peaks. However, referring to
  • a method of manufacturing a radio frequency tag comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor, wherein the inductance of the coil and/or the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof, so as to randomize the respective resonant response of said at least one of said resonator circuits; and coupling said resonator circuits such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
  • the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof.
  • the method may comprise the step of capacitively coupling said at least two resonator circuits.
  • the at least two resonator circuits are capacitively coupled by means of a capacitor having a capacitance randomized during fabrication thereof.
  • the capacitance of said capacitor of said at least one of said resonant circuits and/or the capacitance providing the coupling between the at least two resonant circuits is randomized during fabrication thereof by providing in the dielectric layer a plurality of randomised particles with non-linear properties (i.e. dieliectric constants).
  • the capacitance can be randomized during fabrication thereof by providing in the dielectric layer a plurality of particles having a variable dielectric constant.
  • the dielectric constant of said particles may be dependent upon the strength of an external magnetic field, or the particles may comprise a material having a nonlinear dielectric constant, or the particles may be piezoelectric, or the particles may comprise a phase change material.
  • the capacitance may be randomized by randomizing a thickness of a dielectric layer thereof.
  • the dielectric layer may be a photo-sensitive layer and said thickness thereof is randomized by exposure thereof to radiation of a randomly varying dose, and/or the shape of the electrodes can be randomised.
  • the present invention extends to a radio frequency identification tag manufactured according to the method defined above, comprising at least two resonator circuits, at least one of said resonator circuits having a randomized resonant response, said resonator circuits being coupled such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
  • the at least two resonator circuits are preferably capacitively coupled, beneficially by means of a randomized capacitance.
  • the coils of each of said resonator circuits are arranged and configured such that the fluxes thereof, generated by an external magnetic field when in use, at least partially cancel each other out.
  • the tag may comprise two opposing metal layers, wherein a coil is provided in each of said metal layers and configured such that said coils partially overlap to such an extent that their mutual inductive coupling is substantially zero.
  • the two overlapping coils may be provided on a single layer (the crossing points being isolated from each other by means of vias).
  • the tag may comprise first and second coplanar coils in a single metal layer, the coils being configured such that one of said coils is located within the other.
  • the second coil may comprise at least two windings, one of said windings being located adjacent said first coil and the second winding extending around said first coil.
  • a method of manufacturing a radio frequency identification tag comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor; and capacitively coupling said resonator circuits by means of a capacitance randomized during fabrication thereof such that the resonant responses of said resonant circuits, in combination, represent a unique identifier in respect of said tag.
  • the measured response can be measured using a single antenna covering the whole tag.
  • the tag comprises a substrate having two opposing metal layers, wherein a coil is provided on each of said metal layers and configured such that said coils partially overlap to such an extent that their mutual inductive coupling is substantially zero.
  • the tag comprises a substrate, wherein first and second coplanar coils are provided on the same metal layer thereof, the coils being configured such that one of said coils is located within the other of said coils.
  • the second coil comprises at least two windings, one of said windings being located adjacent said first coil and a second winding extending around said first coil. The second winding has the effect of capturing more of the magnetic flux.
  • a radio frequency identification tag comprising at least two resonator circuits, at least one of said resonator circuits having a random resonant response, said at least two resonator circuits being coupled by a capacitive coupling.
  • the capacitive coupling is random.
  • a measured response is preferably obtained from each resonator circuit separately, and each response will include a distinct peak height ratio in relation to the capacitor of the resonator circuit and the random capacitive coupling.
  • the manner in which the measured response is obtained may comprise the use of a pickup antenna covering both of the tag coils simultaneously, or a pickup antenna covering one of the coils and then the other. It is possible to combine the results of these three measurements.
  • random capacitance may be achieved by means of, for example, a random layer thickness, a random area between the plates covered with dielectric, a random dielectric constant (achieved by using a random mixture of particles that have different dielectric properties), a randomized shape of the capacitor electrodes.
  • a random layer thickness a random area between the plates covered with dielectric
  • a random dielectric constant a random mixture of particles that have different dielectric properties
  • the coils could, instead or additionally, have a random inductance, as mentioned above.
  • Fig. Ia is a graphical representation of a normalized measurement signal from uncoupled oscillators for a measurement coil that couples two antennas with equal strength;
  • Fig. Ib is a graphical representation of a normalized measurement signal from coupled oscillators for a measurement coil that couples two antennas with equal strength;
  • Fig. 2a is a schematic plan view of a tag according to an exemplary embodiment of the first aspect of the invention, having overlapping winding as on opposing metal layers;
  • Fig. 2b is a schematic side view of the tag of Fig. 2a;
  • Fig. 3 is a schematic circuit diagram of an tag according to an exemplary embodiment of the second aspect of the invention;
  • Figs. 4a and 4b are respective schematic diagrams illustrating exemplary realizations of the circuit of Fig. 3;
  • Figs. 6a and 6b are schematic diagrams illustrating two alternative exemplary embodiments of the first aspect of the present invention.
  • Fig. 7 is a schematic cross-sectional diagram illustrating a capacitor with local variation in thickness of dielectric.
  • a tag which comprises a substrate having opposing metal layers, wherein a first winding or coil 10 is provided in one of the metal layers and a second winding or coil 12 is provided in the other metal layer.
  • the coils 10, 12 are configured to partially overlap such that the inductive coupling therebetween is substantially zero, as will now be explained in more detail below.
  • the overlapping coils may be provided in the same metal layer, with crossing points isolated from each other by means ofvias.
  • a 'large' measurement coil may be used to obtain the measured response, that covers both antennas 10, 12 and still yields two well distinguishable peaks, as shown in Figure Ia.
  • FIG. 3 A second exemplary embodiment of the invention is depicted in Figure 3 of the drawings.
  • the resonators are designed such that they have a capacitive mutual coupling C c .
  • This coupling is random (just like the capacitances Ci and C 2 ), which makes it harder to clone the whole circuit because a counterfeiter would not only have to make forgeries of the individual resonators, but also couple them in the correct way.
  • the measurement coil has approximately the size of one resonator. First the coil is placed near the first resonator and a frequency sweep is made, resulting in a response curve Si( ⁇ ) as shown in Figure 5b. Then this procedure is performed in respect of the second resonator, giving a response curve S 2 ( ⁇ ) as shown in Figure 5c. If the mutual coupling is weak, then each curve will have one high peak and one low peak. If the coupling is strong, each curve will have two high peaks. From the two curves, an identifier is derived. Two possible configurations of the circuit of Figure 3 are illustrated in Figures 4a and 4b respectively, wherein the readout coil 20 is also shown.
  • a third exemplary embodiment of the invention can be applied.
  • use is made of the same principle as that applied to the embodiment described in relation to Figure 2 in the sense that the flux generated by a coil is positive inside the coil but negative on the outside. Most of the negative flux is located near the coil.
  • the outer coil 16 has an extra winding so as to capture more of the magnetic flux.
  • the capacitor may include a low concentration of particles with a special property, so as to randomize the capacitance of each of a batch of capacitors manufactured during a single process.
  • the particles may have a B-(magnetic field)- dependent dielectric constant ⁇ r .
  • a detection device for measuring the response curves
  • the addition in the dielectric of the extra particles causes a set of B-dependent shifts of ⁇ , which set can be translated to extra identifying bits in respect of the tag.
  • magneto-electric particles it is also possible to use a nonlinear dielectric (i.e. randomised particles with non-linear properties) for the same purpose.
  • This kind of substance known to a person skilled in the art, has an ⁇ r that depends on the electric field strength, i.e. on the voltage applied to the capacitor plates.
  • the detector in this case, would do a frequency sweep at a number of different power levels. The higher the power in the RF field, the higher the voltage across the capacitor in the LC-PUF. Each frequency sweep effectively detects a different capacitance, and these differences give extra information that can be encoded as extra bits in the LC-PUF identifier.
  • piezoelectric particles are used.
  • the capacitor voltage causes a mechanical stress in the particles which deforms the capacitor. The deformation affects the capacitance.
  • the use of such particles has the effect of randomizing the capacitance.
  • the particles could consist of a material that can be induced to change between a crystalline and amorphous state (i.e. a phase change material).
  • the dielectric constant of the particles in this case, depends on the state. Examples of this type of material include GeSbTe and AgInSbTe alloys, which are also used for optical recording.
  • the detector in this case, would comprise means to effect a phase change (e.g. a laser).
  • the detector would then measure a resonant response when the particles are in a first phase, then cause a phase change, and then measure the response again. Finally, it changes the phase back to its original state. From the two different states, more bits of data can be extracted than from a single response.
  • the capacitor has a parallel plate geometry, one of the plates is optionally made of a transparent material (e.g. ITO) so that the laser beam can reach the particles more easily.
  • a transparent material e.g. ITO
  • a random capacitance value can be obtained by introducing an extra dielectric layer with random thickness on top of the standard dielectric layer between the electrodes of the capacitor, as shown schematically in Figure 7 of the drawings.
  • the thickness of the random dielectric can vary between 0 and roughly 100 times the thickness of the standard dielectric layer.
  • the extra dielectric layer may be a photosensitive layer and the remaining thickness after exposure and development would depend on the actual exposure dose used.
  • local variation of the exposure dose will lead to local variation in thickness of the dielectric layer, and such local variation of the exposure dose can be obtained by modulating the amplitude and phase of a (partially) coherent UV beam.

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  • Computer Security & Cryptography (AREA)
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Abstract

A radio frequency identification (RFID) tag, wherein at least two resonators circuits having random resonant responses are provided and the response curves thereof measured to generate a unique identifier in respect of the tag. In one configuration, the coils of two resonator circuits are provided on opposing metal layers, and arranged such that they overlap relative to each other to the extent that the inductive coupling is substantially zero. In an alternative configuration, the tag comprises two coplanar coils, one located within the other. In a third configuration, two resonator circuits are randomly capacitively coupled.

Description

RF identification tag
FIELD OF THE INVENTION
This invention relates to radio frequency (RF) identification tag having a random resonant frequency that serves as a unique identifier.
BACKGROUND OF THE INVENTION
Counterfeiting is a serious problem affecting many areas of industry. The severity of the problem is reflected in the large number of anti-counterfeiting (AC) technologies available, including security inks, gratings, holograms, magnetic dots, fluorescent and phosphorescent materials, watermarks, rare chemicals, electromagnetic codes, etc. In this regard, it is highly desirable to have cheap authenticity markers that can be generically applied to broad ranges of products.
Radio Frequency Identification (RFID) is increasingly being employed as an AC measure. However, for most applications, RFID tags are currently too costly for item- level tagging. A very promising alternative is the use of so-called chipless RFID, i.e. completely passive tags that do not contain an integrated circuit. However, chipless RFID is a track-and-trace technology vulnerable to cloning of the tags.
On the other hand, it has been recognized that the authenticity marker should be something that cannot be controlled by its maker (hereinafter referred to as "manufacturer- resistance"). Ideally, the marker is a feature intrinsic to the product that has to be protected, but when this is not achievable, the product can be tagged with an unclonable marker. A simple resonator circuit (also known as a 'tank circuit' or LC-circuit) consists of a coil (with self- inductance L) and a capacitor (with capacitance C). The resonant frequency, defined as that frequency where the absolute value of the impedance has a minimum, is given by ω=l/V(LC). When placed in an electromagnetic field whose magnetic part couples well into the coil, the circuit will absorb a frequency-dependent amount of power from the field, with a peak at ω, and this can be measured by a measurement device. The inductance of a coil is a complicated function of the coil geometry. The capacitance of a parallel plate capacitor is dependent on the plate area, the relative dielectric constant of the material between the plates and the separation between the plates. It will thus be apparent that that the resonant frequency of the circuit can be randomized by making any of the parameters affecting capacitance or inductance random.
For technical reasons, in the following, it is assumed that the capacitance of each of a number of resonators is made random, by randomizing the separation between the plates and/or the dielectric constant of the dielectric layer between the plates, and that the inductance L is fixed, thereby resulting in a respective random resonant frequency which serves as a unique identifier and can be measured by a detection device. However, the specific embodiments described hereinafter are not intended to be limited in this regard, and it will be appreciated that the resonant frequency of the circuits can equally be made random by changing other relevant parameters.
The unique identifier thus obtained for each tag can be stored in a secure database or certified by a Certification Authority. Items that do not carry a valid tag (i.e. listed in the database or a certificate) are considered counterfeits. In order to create a counterfeit that passes verification, an attacker must either make a precise physical clone of the random resonator or forge a digital signature, or modify the information in a secure database.
The level of difficulty which a counterfeiter would encounter in forging a tag tends to be at least partially proportional to the number of bits in the respective unique identifier. For large scale systems with millions if distinct tags, an identification capacity of greater than 20 bits is required. In practice, a single LC-circuit cannot achieve an identification capacity of 20 bits. Therefore, in order to achieve such an identification capacity, multiple LC-circuits must be used. US Patent no. 6,304,169 describes an RFID tag in which two or more LC-circuits are cross-coupled such that the unique identifier is derived from the resultant two or more respective resonant frequencies. The circuits need to be cross- coupled, otherwise a counterfeiter could simply forge each resonator separately, i.e. n tags do not offer significantly more protection than one. This problem can be mitigated to a certain extent by placing multiple resonators close together in the same mechanical constructions. However, the main drawback of this approach is that the resonance peaks become less distinct. Referring to Figure Ia of the drawings, the frequency response from two completely separate LC circuits has the form of two distinct resonance peaks. However, referring to
Figure Ib of the drawings, putting them closer together inevitably causes inductive coupling between them, leading to the illustrated response, and one of the peaks is so small that it is lost in the measurement noise, which makes it difficult to extract all of the required information from the response. SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a low-cost RFID tag, in which the identification capacity is sufficiently high to prevent cloning, and the measured response signal is of a sufficiently high quality for the required data to be extracted therefrom.
In accordance with a first aspect of the present invention, there is provided, a method of manufacturing a radio frequency tag, comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor, wherein the inductance of the coil and/or the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof, so as to randomize the respective resonant response of said at least one of said resonator circuits; and coupling said resonator circuits such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
Beneficially, the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof.
In one exemplary embodiment, the method may comprise the step of capacitively coupling said at least two resonator circuits. In this case, beneficially, the at least two resonator circuits are capacitively coupled by means of a capacitor having a capacitance randomized during fabrication thereof.
The capacitance of said capacitor of said at least one of said resonant circuits and/or the capacitance providing the coupling between the at least two resonant circuits is randomized during fabrication thereof by providing in the dielectric layer a plurality of randomised particles with non-linear properties (i.e. dieliectric constants). The capacitance can be randomized during fabrication thereof by providing in the dielectric layer a plurality of particles having a variable dielectric constant. The dielectric constant of said particles may be dependent upon the strength of an external magnetic field, or the particles may comprise a material having a nonlinear dielectric constant, or the particles may be piezoelectric, or the particles may comprise a phase change material.
Alternatively, or in addition, the capacitance may be randomized by randomizing a thickness of a dielectric layer thereof. In this case, the dielectric layer may be a photo-sensitive layer and said thickness thereof is randomized by exposure thereof to radiation of a randomly varying dose, and/or the shape of the electrodes can be randomised. The present invention extends to a radio frequency identification tag manufactured according to the method defined above, comprising at least two resonator circuits, at least one of said resonator circuits having a randomized resonant response, said resonator circuits being coupled such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
In one exemplary embodiment, the at least two resonator circuits are preferably capacitively coupled, beneficially by means of a randomized capacitance.
Alternatively, the coils of each of said resonator circuits are arranged and configured such that the fluxes thereof, generated by an external magnetic field when in use, at least partially cancel each other out. In one exemplary embodiment, the tag may comprise two opposing metal layers, wherein a coil is provided in each of said metal layers and configured such that said coils partially overlap to such an extent that their mutual inductive coupling is substantially zero. Alternatively, the two overlapping coils may be provided on a single layer (the crossing points being isolated from each other by means of vias). In another exemplary embodiment, the tag may comprise first and second coplanar coils in a single metal layer, the coils being configured such that one of said coils is located within the other. In this case, the second coil may comprise at least two windings, one of said windings being located adjacent said first coil and the second winding extending around said first coil. In accordance with a second aspect of the present invention, there is provided a method of manufacturing a radio frequency identification tag, comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor; and capacitively coupling said resonator circuits by means of a capacitance randomized during fabrication thereof such that the resonant responses of said resonant circuits, in combination, represent a unique identifier in respect of said tag.
By arranging the cross-coupled circuits specifically to minimize inductive coupling therebetween, any reduction in the quality of the measured response can also be minimized, whilst the measured response is still dependent on the component properties of both circuits. In this case, the measured response, including two (or more) distinct peaks, can be measured using a single antenna covering the whole tag.
In one exemplary embodiment of the invention, the tag comprises a substrate having two opposing metal layers, wherein a coil is provided on each of said metal layers and configured such that said coils partially overlap to such an extent that their mutual inductive coupling is substantially zero.
In an alternative exemplary embodiment, the tag comprises a substrate, wherein first and second coplanar coils are provided on the same metal layer thereof, the coils being configured such that one of said coils is located within the other of said coils. In one exemplary embodiment, the second coil comprises at least two windings, one of said windings being located adjacent said first coil and a second winding extending around said first coil. The second winding has the effect of capturing more of the magnetic flux.
In accordance with a third aspect of the present invention, there is provided a radio frequency identification tag, comprising at least two resonator circuits, at least one of said resonator circuits having a random resonant response, said at least two resonator circuits being coupled by a capacitive coupling.
Beneficially, the capacitive coupling is random.
In this case, in order to clone the entire tag, a counterfeiter would not only have to clone the individual resonator circuits, but also couple them in the correct manner. A measured response is preferably obtained from each resonator circuit separately, and each response will include a distinct peak height ratio in relation to the capacitor of the resonator circuit and the random capacitive coupling.
It will be appreciated that, in relation to both of the above-mentioned aspects, the manner in which the measured response is obtained may comprise the use of a pickup antenna covering both of the tag coils simultaneously, or a pickup antenna covering one of the coils and then the other. It is possible to combine the results of these three measurements.
In the case of both aspects of the invention, random capacitance (or the random resonant frequency) may be achieved by means of, for example, a random layer thickness, a random area between the plates covered with dielectric, a random dielectric constant (achieved by using a random mixture of particles that have different dielectric properties), a randomized shape of the capacitor electrodes. Other options will be a apparent to a person skilled in the art. Furthermore, the coils could, instead or additionally, have a random inductance, as mentioned above. These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein. BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:
Fig. Ia is a graphical representation of a normalized measurement signal from uncoupled oscillators for a measurement coil that couples two antennas with equal strength;
Fig. Ib is a graphical representation of a normalized measurement signal from coupled oscillators for a measurement coil that couples two antennas with equal strength;
Fig. 2a is a schematic plan view of a tag according to an exemplary embodiment of the first aspect of the invention, having overlapping winding as on opposing metal layers;
Fig. 2b is a schematic side view of the tag of Fig. 2a; Fig. 3 is a schematic circuit diagram of an tag according to an exemplary embodiment of the second aspect of the invention;
Figs. 4a and 4b are respective schematic diagrams illustrating exemplary realizations of the circuit of Fig. 3;
Figs. 5a, 5b and 5c illustrate responses from the circuit shown in Fig. 3, as a function of ω, wherein R=4Ω, L=5nH, Ci=7pF and C2=5pF, and the measurement on the vertical axis is |Ztot-Zo|/ω2, where Ztot is the total impedance and Zo is the impedance of the measurement circuit: Fig. 5a illustrates the response from the left resonator in the case where C12=O, Fig. 5b illustrates the response from the left resonator in the case where Ci2=8pF and Fig. 5c illustrates the response from the right resonator in the case where Ci2=8pF;
Figs. 6a and 6b are schematic diagrams illustrating two alternative exemplary embodiments of the first aspect of the present invention; and
Fig. 7 is a schematic cross-sectional diagram illustrating a capacitor with local variation in thickness of dielectric.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Referring to Figures 2a and 2b of the drawings, in a first exemplary embodiment of the invention, a tag is provided which comprises a substrate having opposing metal layers, wherein a first winding or coil 10 is provided in one of the metal layers and a second winding or coil 12 is provided in the other metal layer. The coils 10, 12 are configured to partially overlap such that the inductive coupling therebetween is substantially zero, as will now be explained in more detail below. Alternatively, the overlapping coils may be provided in the same metal layer, with crossing points isolated from each other by means ofvias.
Two non-overlapping coplanar windings have a negative inductive coupling L12. The exact value follows from the shapes of the windings and the distance separating them. On the other hand, two identical windings that completely overlap have the largest possible inductive coupling, Li2 = +L, where L is the self-inductance. This is a positive value. Therefore, it follows that there exists a partially overlapping state such that the coupling is substantially zero.
In the configuration shown in Figure 2, a 'large' measurement coil may be used to obtain the measured response, that covers both antennas 10, 12 and still yields two well distinguishable peaks, as shown in Figure Ia.
A second exemplary embodiment of the invention is depicted in Figure 3 of the drawings. The resonators are designed such that they have a capacitive mutual coupling Cc. This coupling is random (just like the capacitances Ci and C2), which makes it harder to clone the whole circuit because a counterfeiter would not only have to make forgeries of the individual resonators, but also couple them in the correct way.
In this case, the following detection method is proposed. The measurement coil has approximately the size of one resonator. First the coil is placed near the first resonator and a frequency sweep is made, resulting in a response curve Si(ω) as shown in Figure 5b. Then this procedure is performed in respect of the second resonator, giving a response curve S2(ω) as shown in Figure 5c. If the mutual coupling is weak, then each curve will have one high peak and one low peak. If the coupling is strong, each curve will have two high peaks. From the two curves, an identifier is derived. Two possible configurations of the circuit of Figure 3 are illustrated in Figures 4a and 4b respectively, wherein the readout coil 20 is also shown.
When there is no alternative to placing all of the coils in the same metal layer, a third exemplary embodiment of the invention can be applied. In this case, use is made of the same principle as that applied to the embodiment described in relation to Figure 2 in the sense that the flux generated by a coil is positive inside the coil but negative on the outside. Most of the negative flux is located near the coil. Thus, by placing a loop 16 around the coil 14, placed at a moderate distance therefrom, will have a low magnetic coupling to the coil 14, as shown schematically in Figures 6a and 6b of the drawings. In the configuration shown in Figure 6b, the outer coil 16 has an extra winding so as to capture more of the magnetic flux. In all cases, it is preferred (from a fabrication point of view) to randomize the capacitance - of at least one of the resonator circuits and/or the capacitive coupling (in relation to the second embodiment described above).
In one proposed method, the capacitor may include a low concentration of particles with a special property, so as to randomize the capacitance of each of a batch of capacitors manufactured during a single process.
In a first exemplary embodiment, the particles may have a B-(magnetic field)- dependent dielectric constant εr. In this case, a detection device (for measuring the response curves) would be configured to apply a number of different external field strengths and for each field strength measure the resonant frequency ω. Thus, the addition in the dielectric of the extra particles causes a set of B-dependent shifts of ω, which set can be translated to extra identifying bits in respect of the tag.
As an alternative to magneto-electric particles, it is also possible to use a nonlinear dielectric (i.e. randomised particles with non-linear properties) for the same purpose. This kind of substance, known to a person skilled in the art, has an εr that depends on the electric field strength, i.e. on the voltage applied to the capacitor plates. The detector, in this case, would do a frequency sweep at a number of different power levels. The higher the power in the RF field, the higher the voltage across the capacitor in the LC-PUF. Each frequency sweep effectively detects a different capacitance, and these differences give extra information that can be encoded as extra bits in the LC-PUF identifier.
Yet another option is to use piezoelectric particles. The capacitor voltage causes a mechanical stress in the particles which deforms the capacitor. The deformation affects the capacitance. Thus, the use of such particles has the effect of randomizing the capacitance. Finally, the particles could consist of a material that can be induced to change between a crystalline and amorphous state (i.e. a phase change material). The dielectric constant of the particles, in this case, depends on the state. Examples of this type of material include GeSbTe and AgInSbTe alloys, which are also used for optical recording. The detector, in this case, would comprise means to effect a phase change (e.g. a laser). The detector would then measure a resonant response when the particles are in a first phase, then cause a phase change, and then measure the response again. Finally, it changes the phase back to its original state. From the two different states, more bits of data can be extracted than from a single response. If the capacitor has a parallel plate geometry, one of the plates is optionally made of a transparent material (e.g. ITO) so that the laser beam can reach the particles more easily.
In yet another method, a random capacitance value can be obtained by introducing an extra dielectric layer with random thickness on top of the standard dielectric layer between the electrodes of the capacitor, as shown schematically in Figure 7 of the drawings. The thickness of the random dielectric can vary between 0 and roughly 100 times the thickness of the standard dielectric layer.
In one exemplary embodiment, the extra dielectric layer may be a photosensitive layer and the remaining thickness after exposure and development would depend on the actual exposure dose used. Thus, local variation of the exposure dose will lead to local variation in thickness of the dielectric layer, and such local variation of the exposure dose can be obtained by modulating the amplitude and phase of a (partially) coherent UV beam.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:
1. A method of manufacturing a radio frequency tag, comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor, wherein the inductance of the coil and/or the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof, so as to randomize the respective resonant reponse of said at least one of said resonator circuits; and coupling said resonator circuits such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
2. A method according to claim 1, wherein the capacitance of the capacitor of at least one of said resonator circuits is randomized during fabrication thereof, and/or comprising the step of capacitively coupling said at least two resonator circuits, optionally by means of a capacitor having a capacitance randomized during fabrication thereof.
3. A method according to claim 2, wherein the capacitance of said capacitor of said at least one of said resonant circuits and/or the capacitance used to capacitively couple said resonant circuits is randomized during fabrication thereof by providing in the dielectric layer a plurality of randomised particles with non-linear properties.
4. A method according to claim 3, wherein the dielectric constant of said particles is dependent upon the strength of an external magnetic field.
5. A method according to claim 3, wherein said particles comprise a material having a nonlinear dielectric constant.
6. A method according to claim 3, wherein said particles are piezoelectric.
7. A method according to claim 3, wherein said particles comprise a phase change material.
8. A method according to claim 2, wherein said capacitance is randomized by randomizing a thickness of a dielectric layer thereof.
9. A method according to claim 8, wherein said dielectric layer is a photosensitive layer and said thickness thereof is randomized by exposure thereof to radiation of a randomly varying dose.
10. A radio frequency identification tag manufactured according to the method of claim 1, comprising at least two resonator circuits, at least one of said resonator circuits having a randomized resonant response, said resonator circuits being coupled such that the resonant responses thereof, in combination, represent a unique identifier in respect of said tag.
11. A tag according to claim 10, wherein said at least two resonator circuits are capacitively coupled, and optionally wherein said at least two resonator circuits are capacitively coupled by a randomized capacitance.
12. A tag according to claim 11 , wherein the coils of each of said resonator circuits are arranged and configured such that the fluxes thereof, generated by an external magnetic field when in use, at least partially cancel each other out.
13. A tag according to claim 12, comprising two opposing metal layers, wherein a coil is provided in each of said metal layers and configured such that said coils partially overlap to such an extent that their mutual inductive coupling is substantially zero.
14. A tag according to claim 13, comprising first and second coplanar coils in a single metal layer, the coils being configured such that one of said coils is located within the other, and optionally wherein the second coil comprises at least two windings, one of said windings being located adjacent said first coil and the second winding extending around said first coil.
15. A method of manufacturing a radio frequency identification tag, comprising: providing at least two resonator circuits, each consisting of at least one inductive coil and one capacitor; and capacitively coupling said resonator circuits by means of a capacitance randomized during fabrication thereof such that the resonant responses of said resonant circuits, in combination, represent a unique identifier in respect of said tag.
PCT/IB2009/050959 2008-03-14 2009-03-09 Rf identification tag WO2009112999A1 (en)

Applications Claiming Priority (2)

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EP08152787 2008-03-14
EP08152787.1 2008-03-14

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20010040507A1 (en) * 2000-05-08 2001-11-15 Checkpoint Systems, Inc. Radio frequency detection and identification system
WO2007072251A2 (en) * 2005-12-22 2007-06-28 Koninklijke Philips Electronics N.V. Security element and methods for manufacturing and authenticating the same

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20010040507A1 (en) * 2000-05-08 2001-11-15 Checkpoint Systems, Inc. Radio frequency detection and identification system
WO2007072251A2 (en) * 2005-12-22 2007-06-28 Koninklijke Philips Electronics N.V. Security element and methods for manufacturing and authenticating the same

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