WO2006039805A1

WO2006039805A1 - Radio-frequency identification tag

Info

Publication number: WO2006039805A1
Application number: PCT/CA2005/001569
Authority: WO
Inventors: Nicolas Schieli
Original assignee: Quelis Id Systems Inc.
Priority date: 2004-10-14
Filing date: 2005-10-13
Publication date: 2006-04-20

Abstract

A radio-frequency identification (RFID) tag comprises a substrate, a microchip mounted to the substrate, a coupling coil in the form of one turn of wire printed on the substrate or etched from an electrically conductive material and terminated by two terminal ends connected to the microchip, and coil antenna, self-resonating at the operative frequency of the tag, mounted on top of the coupling coil so as to be inductively coupled thereto. According to a further embodiment, the coupling coil is in the form of a two turn wire wherein the chip is connected is further connected at a middle point between the two terminal ends. This second tag arrangement allows achieving a power transfer identical to a Graetz bridge with only two diodes.

Description

TITLE OF THE INVENTION

RADIO-FREQUENCY IDENTIFICATION TAG

FIELD OF THE INVENTION

The present invention relates to radio-frequency identification (RFID) transponders.

More specifically, the present invention relates to RFID tags.

BACKGROUND OF THE INVENTION

Conventional radio-frequency identification (RFID) transponders are usually made of an antenna engraved or printed on a substrate with a microchip attached to it using known techniques. This conventional RFID manufacturing process allows for very high throughputs and yields. Indeed, current technology such as flip-chip allows reaching typical capacities of 10 000 units per hour. However, some limitations exist. For example, a 13.56 MHz antenna is particularly difficult to produce at a very low price using the above-described process, especially of small form factor, which implies significant drawbacks to the electric characteristics.

Figure 1 of the appended drawings illustrates a conventional

RFID tag 2. The tag 2 comprises a microchip 4 and a coil antenna 6 connected thereto, both mounted on a substrate 8. The microchip 4 and antenna 6 are secured to the substrate 8 using well-known techniques as described hereinabove. The tag can be further processed including being laminated or moulded in plastic.

The microchip typically includes a resonance capacitor, a clock extractor, a voltage rectifier, a modulation stage and a memory associated with all circuitry required to access its content.

Different manufacturing techniques have been developed over the last years to optimize such a design. Obviously tag manufacturers search to reduce manufacturing costs of every part and to focus on increasing throughputs.

A process involving the use of a standard screen-printing technique and an electrically conductive ink has been proposed to print antenna which are then directly connected to a bare microchip. The direct connexion is done at very high speed using any of well-known techniques such as flip-chip. Finally, the microchip is fastened to the coil and its underlying substrate by means of, for instance, anisotropic glue. Other connecting techniques may be used with different throughputs and costs.

However, limitations of conventional tags include:

• the conductivity of the printed antenna is low, therefore causing the dissipation of a large portion of incident electro-magnetic (EM) power;

© the self-inductance of very small printed antenna is below 1uH. It requires large tuning capacity on chip and induces extra costs. Therefore, its accuracy is difficult to guarantee in production; • the resolution and pitch of printed antenna are not compatible with very small form factors; and

• the printed antennas are usually formed by 3 to 10 turns. It is impossible to form such arrangement on a single layer. Therefore, traditional techniques close the coil circuit by means of a strap, resulting in an additional cost.

A RFID tag free of the above-mentioned limitations is therefore desirable.

OBJECTS OF THE INVENTION

An object of the present invention is therefore to provide improved radio-frequency identification transponders and tags.

SUMMARY OF THE INVENTION

More specifically, in accordance with a first aspect of the present invention, there is provided a radio-frequency identification (RFID) tag characterized by an operational frequency comprising: a substrate; a microchip mounted to the substrate; a coupling coil mounted to the substrate including at least one turn of wire terminated by two terminal ends connected to the microchip; and a coil antenna mounted on top of the coupling coil so as to be inductively coupled thereto; the coil antenna being electrically closed by a parasitic capacitance distributed along its coil; the parasitic capacitance causing the coil antenna to self-resonate at the operational frequency of the tag.

According to a second aspect of the present invention, there is provided a radio-frequency identification (RFID) tag manufacturing process comprising: securing at least one turn of an electrically conductive wire to a substrate, yielding a coupling coil having terminal ends; securing a microchip to the substrate; connecting the two terminal ends to the microchip; providing a coil antenna characterized by a parasitic capacitance providing a self-resonance at an operational frequency of the tag; the coil antenna being electrically closed by the parasitic capacitance; and mounting the coil antenna on top of the coupling coil.

It is to be noted that the terms "chip" or "microchip" are not intended herein in a limited way. They should be construed so as to include any miniaturized processing device.

Other objects, advantages and features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings: Figure 1 , which is labeled "prior art" is a radio-frequency identification tag according to an arrangement from the prior art;

Figure 2 is a partially exploded perspective view of a radio- frequency identification (RFID) tag according to a first illustrative embodiment of the present invention;

Figure 3 is a top plan view of the radio-frequency identification (RFID) tag from Figure 2, illustrating the tag assembled;

Figures 4A-4C are R-L-C circuits modelizing radio-frequency identification (RFID) tags according to illustrative embodiments of the present invention;

Figure 5 is a partially exploded perspective view of a radio- frequency identification (RFID) tag according to a second illustrative embodiment of the present invention;

Figure 6 is a medicine bottle incorporating the RFID tag from

Figure 2 as a safety feature; and

Figure 7 is a flowchart of a tag manufacturing process according to an illustrative embodiment of the present invention.

DETAILED DESCRIPTION

A radio-frequency identification (RFID) tag 10 according to a first illustrative embodiment of the present invention will now be described with reference to Figures 2 and 3.

The tag 10 comprises a substrate 12, a microchip 14 and a coupling coil 16 both mounted to the substrate 12, and a coil antenna 18 mounted on top of the coupling coil 16 so as to be inductively coupled thereto. The coupling coil 16 allows to inductively coupling the antenna 18 to the microchip 14 while being disconnected therefrom as will be explained hereinbelow in more detail.

The microchip 14 is secured to the substrate 12 using one of many techniques well-known in the art, such as fastening with an anisotropic conductive film or via addition of bumps on the chip's pads including solder bumps, gold bumps, etc.

The coupling coil 16 includes at least one turn of wire terminated by two terminal ends 20 connected to the microchip 14. Even though more than one turn of wire can be provided in forming the coil 16, it has been found that providing only one turn is both sufficient and obviously more economical.

The coupling coil 16 can be for example printed on the substrate 12 or etched from an electrically conductive material such as, but not limited to copper. Indeed, any other electrically conductive material can of course be used.

The connection between the microchip 14 and the terminal ends 20 of the coupling coil 16 may be made through flip-chip technique for instance as it is particularly cost-efficient. Of course, other known connecting techniques can also be used.

The wound coil antenna 18 is made of fine insulated electrical conductor. This coil 18 is configured and sized to carry a parasitic capacitance making it self-resonate at the operational frequency of the tag, which can be 13.56MHz for example.

The coil antenna 18 is made of wound wire having its terminals not connected. The antenna effect is due to its parasitic capacitance distributed along the conductor. The antenna 18 circuit is electrically closed by the distributed parasitic capacitance along its wiring. This allows avoiding any mechanical connection during the manufacturing process reducing the total cost to its lowest.

It is to be noted that the antenna 18 is illustrated only schematically in Figures 2 and 3. More specifically, the number of turns of wire illustrated does not necessarily reflect the reality.

To maximize the coupling between the two coils 16 and 18, they are made concentric and of approximately the same diameter. This allows achieving a very high mutual coupling of 80 percent or more.

In operation, the wound coil 18 acts as a regular RFID antenna amplifying the induced current from a reader (not shown). This current creates in turn an induced voltage in the single loop coil 16 powering up the microchip 14. According to the tag arrangement 10, the coupling coil 16 which is directly connected to the chip 14 does not directly recuperate the energy.

As in the case of any wound coil made of an insulated wire, it is well known that an R-L-C circuit can be used to modelize RFID tags. Examples of such circuits modelizing RFID tags according to the present invention are depicted in Figures 4A-4C.

Indeed, plunging the wound coil 18 into a magnetic field creates an induced current l_c in the conductor and a displacement current I_d in the insulation separating each turn. Basically I₀ is flowing through an R-L network and l_d through lumped capacitors.

Therefore, it exists for that coil 18 a resonance frequency for which the current is magnified by a quality factor Q. Being placed right above the coupling coil 16, the coil 18 induces a voltage to the chip 14. Power transfer between both coils 16 and 18 is controlled by standard air- core transformer laws. The wound coil 18 can be seen as primary winding, the microchip-coupling coil 16 as secondary winding and the microchip 14 as the load of the transformer.

The induced voltage in the coupling coil 16 is given by

p prriinntteedd J J ^"^" Λ y/ p prriinntteedd w woouunndd w woowund

^{ω ~ 2}^ , f : electromagnetic field frequency emitted by the reader.

One can see that maximizing the induced voltage to the chip 14 can be achieved by increasing each or any of the following parameter: ^k coupling factor between coils 16-18; ^womd wound coil 18 self-inductance;

^priMed coupling coil 16 self-inductance; wound current flowing along the wound coil 18;

The coupling factor is maximized by optimizing geometries of both coils 16-18.

A good approximation of the coupling factor is given by

i _ ^r wound ^r printed ^C0S "

^■\/ [Z wound ' ^" printed L \ wound ² + d²Y J i

It is to be noted that, in Figures 4A-4C, CF represents a filtering capacitor inherent to the microchip 14.

A tag according to the present invention overcomes most of the traditional tag limitations. Indeed, the coupling coil 16 can be very small as it does only require a single loop. As. its cost is directly related to the amount of conductive ink dispensed, in the case where the coupling coil 16 is printed on the substrate 12 for example, there is a net saving associated with that configuration. Moreover, a chip 14 can be easily connected to it by traditional flip-chip technique without the need of a strap or closing the circuit on another layer reducing the cost further again.

A process 100 for manufacturing a tag according to the present invention is illustrated in Figure 7. It includes the following steps:

102 - securing at least one turn of an electrically conductive wire to a substrate, yielding a coupling coil;

104 - securing a chip to the substrate;

106 - connecting the two terminal ends to the chip; 108 - providing a coil antenna having a diameter similar to the coupling coil diameter and a parasitic capacitance providing a self- resonance at the operational frequency; and

110 - mounting the coil antenna on top of the coupling coil.

The antenna 18 can be manufactured using conventional techniques. It is to be noted that the amount of wire required is very limited (30 to 50 turns depending on internal diameter). The added cost of that component is therefore very limited compared to the advantage of using a flip-chip technique for miniature tags.

Another improvement carried by this arrangement is the relative higher voltage supplied to the chip compared to in a conventional tag. Let note V_p' the chip voltage using that configuration and V_p the same voltage using traditional arrangement.

Given that antenna 18 is self-resonant at the operational frequency, it can be easily demonstrated that Vp' _ kω²L_aC_pR_p g7 Vp " R_a i Lp

with

^ the coupling factor between the antenna 18 and the coupling coil 16; ^a the antenna 18 self-inductance;

D ^a the antenna 18 resistance;

C P the chip 14 capacitor; n ^p the coupling coil resistance in a traditional configuration;

^LP the coupling coil 16 self-inductance; and

P the coupling coil self-inductance in a traditional configuration. Using typical values like,

£ = 0.9 , K = 23μH _^ R_a = l5Ω _^ C_p = 3ApF _^ R_p = A5a _^

Lp' = 20OnF Lp = AμH '

yields

Vp

This higher voltage is particularly interesting in applications involving read/write chips or with high complexity circuits like cryptographic micro- processors for example.

A RFID tag 22 according to a second illustrative embodiment of the present invention will now be described with reference to Figure 5.

Since the tag 22 is similar to the tag 10 illustrated in Figures 2-3, and for concision purposes, only the differences between the two tags 10 and 22, pertaining mainly to the chip connection, will be described herein in more detail.

The tag 22 comprises a two-turn coupling coil 24 terminated by two terminal ends 26 connected to the microchip 14. The microchip 14 is further connected to the coupling coil 24 at another intermediary point, which may be the middle point between the two terminal ends 26.

Microchips extract their power supply from oscillating coil voltage by diode rectification. Traditional arrangement does not allow this middle-tap connection and therefore power supply extraction is done by a single diode (see Figure 4B) or a Graetz Bridge (see Figure 4A). The former case is not very efficient as it gets its power only from one half of the oscillating signal. The later works on the full magnitude, but there are always two diodes conducting at any time, involving a 2V_T voltage drop. Using the middle-tap arrangement illustrated in Figure 5 and modelized in Figure 4C, it is possible to achieve exactly the same power transfer as a Graetz bridge with only two diodes. Then, a single diode is conducting at a given time resulting in a voltage drop of VT only.

Of course, to provide such a tag arrangement, and more specifically a three-point connection between the chip 14 and the coupling coil 24, the chip 14 has to be sufficiently large to receive the three connecting points.

Finally, it has been found that the quality control of a tag according to the present invention can be achieved by controlling the antenna coil 18 thickness (therefore controlling its distributed parasitic resistance along with its parasitic capacitance), which may be particularly interesting for large tags. Indeed, increasing a tag's diameter traditionally results in increasing self-resonating coil's quality factor. To a certain extent the tag behavior may be a net improvement of power transfer, but it also results in less stability of the whole system. Then, it can be very useful to be able to control this quality factor. Except by changing wire diameter (resulting in changing serial resistance), an elegant way of controlling this lies in the self-resonating coil's geometry. Indeed, distributed parasitic capacitance introduces resistive losses in direct relation with the winding's geometrical arrangement. A particularly thick antenna 18 provides a lower quality factor than its thinner counterpart for antennas self-resonating at the same frequency. Thus, by controlling the antenna coil 18 thickness, it is possible to get the desired quality factor for the whole system.

Turning now to Figure 6, a medicine bottle 28 will now be described as an example of an application of an RFID tag according to the present invention. The bottle 28 incorporates an RFID tag 10 as a safety feature as will become more apparent upon reading the following description.

Indeed, a particularly useful arrangement of this invention relates to the fact that a complete tag 10 is formed of two components not directly connected together: a first part including the microchip 14 with the coupling coil 16 connected thereto, both mounted on a substrate 30, and a second part made of the antenna coil 18. It is therefore possible to embed the first part in a plastic and the self-resonating antenna coil 18 in another. Provided that the two parts are affixed together in close proximity, the complete tag will be fully functional. In the case of the medicine bottle 28 illustrated in Figure 6, the second part consisting in the self-resonating coil antenna 18 is embedded in the bottle cap 32 and the first part is sealed in a pealable film 30 also acting as the substrate, which is glued to the opening of the container 34.

In operation, the bottle 28 can be opened either by piercing or pealing the film 30, thereby broking or removing the first part of the tag, adding a tampering functionality to the RFID tag. A tab 36 is advantageously provided on the edge of the film 30 to help pealing.

In the case where the film 30 is to be broken, the coupling coil 16 is positioned near the center of the film 30 to ensure that it will be broken with the film. A pharmacist or an inventory manager would then only have to operate a reader in close proximity to the bottle 28 to verify if the bottle 28 has been tampered with without having to open it.

It is particularly useful to only embed the first part of the tag in the sealing due to its simple manufacturing process and particularly low cost.

A tag according to the present invention can be use similarly in any object having first and a second separable parts; i) the coil antenna being secured to a first part; and ii) the substrate, the microchip and the coupling coil being secured to the second part. Of course, the first and second separable parts are to be mountable in such a way that the tag becomes operable.

Of course, an RFID tag according to the present invention can have many other applications, both more traditional and involving its two-part quality.

Even though the tags according to the present invention have been described without any protection, it is believed to be within the reach of persons having ordinary skills in the art to protect the tag by well-known lamination or resin or plastic molding techniques.

Although the present invention has been described hereinabove byway of preferred embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.

Claims

WHAT IS CLAIMED IS;

1. A radio-frequency identification (RFID) tag characterized by an operational frequency comprising: a substrate; a microchip mounted to said substrate; a coupling coil mounted to said substrate including at least one turn of wire terminated by two terminal ends connected to said microchip; and a coil antenna mounted on top of said coupling coil so as to be inductively coupled thereto; said coil antenna being electrically closed by a parasitic capacitance distributed along its coil; said parasitic capacitance causing said coil antenna to self-resonate at the operational frequency of the tag.

2. A tag as recited in claim 1 , wherein said coupling coil is characterized by a first diameter and said coil antenna is characterized by a second diameter; said first and second diameters being substantially equal.

3. A tag as recited in claim 1 , wherein said coupling coil is printed on said substrate using an electrically conductive ink; at least part of said microchip being bare; said two terminal ends being connected to said microchip via said at least part of said microchip being bare.

4. A tag as recited in claim 1 , wherein said coupling coil is etched on said substrate from an electrically conductive material; at least part of said microchip being bare; said two terminal ends being connected to said microchip via said at least part of said microchip being bare.

5. A tag as recited in claim 1 , wherein said coupling coil is glued to said substrate.

6. A tag as recited in claim 1 , wherein said at least one turn of wire includes a single turn of wire.

7. A tag as recited in claim 1 , wherein said at least one turn of wire includes two turns of wire; said coupling coil being further connected to said microchip at an intermediary point between said two terminal ends.

8. A tag as recited in claim 1 , wherein the operational frequency is 13.56 MHz.

9. A tag as recited in claim 1 , wherein said coil antenna is made of fine insulated electrical conductor.

10. A radio-frequency identification (RFID) tag manufacturing process comprising: securing at least one turn of an electrically conductive wire to a substrate, yielding a coupling coil having terminaj ends; securing a microchip to said substrate; connecting said two terminal ends to said microchip; providing a coil antenna characterized by a parasitic capacitance providing a self-resonance at an operational frequency of the tag; said coil antenna being electrically closed by said parasitic capacitance; and mounting said coil antenna on top of said coupling coil.

11. A process as recited in claim 10, wherein said operational frequency is 13.56MHz.

12. A process as recited in claim 10, wherein said coupling coil is further characterized by a diameter; said coil antenna having a diameter similar to said coupling coil diameter.

13. A process as recited in claim 10, wherein said securing at least one turn of an electrically conductive wire to a substrate includes printing said at least one turn of an electric conductive wire to said substrate using an electrically conductive ink; at least part of said microchip being bare; connecting said two terminal ends to said microchip including connecting said two terminal ends to said microchip via said at least part of said microchip being bare.

14. A process as recited in claim 10, wherein said securing at least one turn of an electrically conductive wire to a substrate includes etching said at least one turn of an electric conductive wire to a substrate from an electrically conductive material; at least part of said microchip being bare; connecting said two terminal ends to said microchip including connecting said two terminal ends to said microchip via said at least part of said microchip being bare.

15. A process as recited in claim 10, wherein said securing at least one turn of an electrically conductive wire to a substrate includes gluing said at least one turn of an electrically conductive wire to said substrate.

16. A process as recited in claim 10, wherein connecting said two terminal ends to said microchip includes the use of a flip-chip technique.

17. A process as recited in claim 10, wherein securing at least one turn of an electrically conductive wire to a substrate includes securing two turns of said electrically conductive wire to said substrate; the process further comprising further connecting said microchip to said conductive wire at an intermediary point between said two terminal ends.

18. A process as recited in claim 10, wherein said securing at least one turn of an electrically conductive wire to a substrate includes securing a single turn of an electrically conductive wire.

19. A process as recited in claim 10, wherein said securing a chip to said substrate includes fastening said chip to said substrate with an anisotropic conductive film.

20. A process as recited in claim 10, further comprising laminating the tag.

21. A process as recited in claim 10, further comprising molding the tag in resin or plastic.

22. The use of the tag from claim 1 in an object having first and a second separable parts; i) said coil antenna being secured to said first part; and ii) said substrate, said microchip and said coupling coil being secured to said second part; said first and second separable parts being mountable such as said tag becomes operable.

23. The use as recited in claim 22, wherein the object is a medicine bottle having an aperture blocked by a seal; said first part being the bottle cap; said second part being said seal.

24. The use as recited in claim 23, wherein said coupling coil is positioned nearer the center of said aperture than the edge of said aperture.

25. The use as recited in claim 23, wherein said seal is a pealable film.