GB2582128A

GB2582128A - Electronic tag

Info

Publication number: GB2582128A
Application number: GB1900667.5A
Authority: GB
Inventors: Mazza Marco
Original assignee: Frisense Ltd
Current assignee: Frisense Ltd
Priority date: 2019-01-17
Filing date: 2019-01-17
Publication date: 2020-09-16
Anticipated expiration: 2039-01-17
Also published as: GB2582128B; GB201900667D0

Abstract

A capacitively coupled radio frequency identification (RFID) tag 10 comprises a semiconductor substrate having first 50 and second 60 planar surfaces, the second distal from the first, a metallic pad formed on the first planar surface, and a circuit formed on the semiconductor substrate and electrically connected to the metallic pad 40 and the second planar surface, the circuit configured to respond to a radio frequency (RF) input signal by providing a data signal encoded by varying an impedance between the metallic pad and the second planar surface of the semiconductor substrate. The data signal may modulate the RF input signal. A method of communicating with a plurality of capacitively coupled RFID tags 10 of the above sort is also claimed, wherein an RF signal is applied to the RFID tags; responding with one RFID tag by varying impedance, said variation encoding a data signal; decoding the data from said variation; and reducing the impedance of the tags that have not responded, while the responsive tag varies its impedance.

Description

Electronic Tag

Field of the Invention

The present invention relates to a capacitive coupled RFID tag and a method for its operation.

Background of the Invention

Capacitive-coupled tags (CC-tag or CCT) provide increased security whilst providing authenticity and tracking functionality. This technology has widespread use for tracking artefacts where security and authenticity is paramount. However, when the size of the CC-tag can be reduced and its efficiency increased then the number of potential applications increases.

Other types of RFID tags exist and these can include antennas for receiving RF signals from a reader to power the device and to respond with a signal using the same antenna. However, use of such antennas can increase the size of the devices and increase the complexity of manufacture and so limit their applications. Furthermore, such antennas can introduce a point of failure resulting in less robust devices. CC-tags do not use an antenna but interact with the RF signal provided by a reader by altering their impedance, which affects the electric field generated by the reader and which in turn is detected by the reader. When such impedance changes are modulated then this modulation can be decoded to provide data (e.g. an identifier of the CC-tag).

"0.075 x 0.075mm2 Ultra-Small 7.5pm Ultra-Thin RFID-Chip Mounting Technology", Hideyuki Noda and Mitsuo Usami, 978-14244-2231-9/08, 2008 IEEE, 2008 Electronic Components and Technology Conference, pages 366 to 370 describes the manufacture of small RFID chips that include an antenna. However, the manufacture of such RFID chips, especially on a large scale, provides technological difficulties, which can reduce the yield in the manufacturing process and increase failure rates.

"Powder RFID Chip Technology", Mitsuo Usami, Hitachi, Ltd., 978-1-4244-2342- 2/08, 2008 IEEE, pages 1220 to 1223 describes another small RFID tag and method of its manufacture, which again requires an antenna structure placed on the RFID chip. This limits the size of the tag to a lower limit.

"26.6 -A 0.05x0.05mm2 RFID Chip with Easily Scaled-Down ID-Memory", Mitsuo Usami, Hisao Tanabe, Akira Sato, !sac) Sakama, Yukio Maki, Toshiaki Iwamatsu, Takashi Ipposhi, Yasuo Inoue, Hitachi, ISSCC 2007/SESSION 26/NON-VOLATILE MEMORIES/26.6, 1-4244-0852-0/07, 2007 IEEE, pages 482 to 483 describes an REID chip that has a unique IF' address and that uses double-surface electrodes.

A further requirement is that RFID tags can be read in the presence of other RFID tags. This can be particularly difficult for very small RFID tags that may be imbedded in many separate items stacked or placed close together.

Therefore, there is required a capacitive coupled tag and method of operation that overcomes these problems.

Summary of the Invention

A capacitive-coupled RFID tag (CC-tag or RFID tag) is provided that is thin (e.g. has a thickness of 50pm or less) is formed on a semiconductor substrate such as a silicon, having one surface covered in a metal layer (e.g. gold on aluminium) and an opposite surface being a bare semiconductor surface, which together act as a tunable impedance. A circuit or chip (e.g. an integrated circuit, IC) on or within the semiconductor substrate controls a device and varies its electrical properties as seen (electrically) from an external reader that applies an electric field through the CC-tag, typically using electrodes. The CC-tag is powered by the externally applied RF electric field and responds to its presence by varying its electrical properties (in particular, its impedance). The CC-tag capacitively couples to the reader and the IC modulates its electrical properties to encode a data signal, which is decoded by the reader. The electrical properties are altered by changing the impedance between the metal layer and the opposite semiconductor layer.

When more than one or a stack of such CC-tags are placed within the electric field generated by the reader, then only one of the CC-tags is configured to respond. The remaining CC-tags reduce their impedance (e.g. statically) by, for example, applying a short circuit between the metal surface and the opposite semiconductor surface, so that each CC-tags in turn and in isolation can respond and provide its output signal by modulating the RF input signal.

There is also provide a method of manufacturing the capacitive-coupled RFID tag by providing a semiconductor substrate (e.g. silicon) having opposing planar surfaces, applying a metallic layer to one of the planar surfaces and forming the circuit described throughout this description (e.g. a CMOS circuit) on or within the other planar surface (e.g. using lithographic techniques). Preferably, the thickness (i.e. distance between the opposing planar surfaces) is equal to or less than 50pm (or 25 pm, 100 pm, or 150 pm). The substrate may be square, rectangular or another shape. Preferably, the substrate and resulting device is between 50 and 700pm wide and/or long.

Application of these concepts includes but is not limited to banknotes, visas, stamps, official documents, holograms, goods and packaging that require such small and micron-thin solutions. A CC-tag may be embedded or bonded to such items.

Against the previously described background and in accordance with a first aspect there is provided a capacitive coupled radio frequency identification, RFID, tag (e.g. a CC-tag) comprising: a semiconductor substrate having a first planar surface and a second planar surface distal from the first planar surface; a metallic pad formed on the first planar surface of the semiconductor substrate; a circuit formed on the semiconductor substrate and electrically connected to the metallic pad and the second planar surface of the semiconductor substrate, the circuit configured to respond to a radio frequency, RF, input signal by providing a data signal encoded by varying an impedance between the metallic pad and the second planar surface of the semiconductor substrate. Having one side providing a metal pad and the other having a semiconductor surface reduces the complexity in manufacture (for example, over CC-tags that may have metal pads on each side) as the semiconductor surface is provided by the substrate, whilst allowing the opposite surfaces to provide a capacitance that can be varied by changing the device impedance between the surfaces.

Preferably, the data signal may be encoded by the varying impedance between the metallic pad and the second planar surface of the semiconductor substrate to modulate the RF input signal. This allows the RF input signal to be used both for powering the device and to be decoded by a reader.

Preferably, the RF input signal may be provided by an external reader.

Advantageously, the circuit is further configured to be powered by the RF input signal.

Advantageously, the circuit may be further configured to decode a signal encoded within the RF input signal and further wherein the data signal is provided in response to the decoded signal. The circuit may optionally include a small amount of power storage (e.g. a capacitor) to store the power generated by the RF input signal for a short time.

Optionally, the circuit modulates the RF input signal by varying its frequency, amplitude and/or phase or any other electrical property.

Optionally, the circuit may be formed on or embedded within the second planar surface of the semiconductor substrate.

Optionally, the circuit may be configured to vary the electrical impedance between the metallic pad and the second planar surface of the semiconductor substrate by applying a short circuit between the metallic pad and the second planar surface of the semiconductor substrate.

Optionally, a distance between an outside surface of the metallic pad and the second planar surface of the semiconductor substrate is equal to or less than 50pm. This may be described as the thickness of the CC-tag. The thickness may alternatively be less than or equal to lOpm, 25pm, 100pm or 150pm, for example. The CC-tag may have a substantially square or rectangular cross-section, for example.

Optionally, the circuit may be further configured to detect the presence of one or more further capacitive coupled RFID tags (e.g. of the same type) and in response, cease providing the data signal. This prevents collision of signals and allows multiple CC-tags to be read sequentially without needing to have only one CC-tag within range of the reader at a time.

Optionally, the circuit may be configured to cease providing the data signal by applying a short circuit between the metallic pad and the second planar surface of the semiconductor substrate. This effectively makes the "off" CC-tag(s) invisible (electrically) to the reader.

Optionally, the circuit may be configured to cease providing the data signal until the one or more further capacitive coupled RFID tags have provided their data signal.

Optionally, the circuit may be further configured to cease providing the data signal according to an anti-collision (i.e. of their data signals) protocol.

Optionally, the anti-collision protocol may be based on communications between the one or more capacitive coupled RFID tags: according to a pre-determined order of response, according to a negotiated response between the one or more capacitive coupled RFID tags, or a random number generator. Such an anti-collision protocol may not be rely on direct communications between CC-tags but these may be through the reader or not rely on communications at all.

Optionally, the first planar surface may be parallel with the second planar surface.

Other configurations may be used provided the surfaces give rise to a capacitance.

Optionally, the capacitive coupled RFID tag is flexible. This may include a bending radius of the order of the width or length of the CC-tag. Therefore, the CC-tag may be embedded or attached to a flexible item with less risk of damage.

In accordance with a second aspect, there is provided item having the capacitive coupled RFID tag according to any previous claim embedded within it.

Optionally, a surface of the item is parallel or substantially parallel with the first planar surface of the capacitive coupled RFID tag. This is useful for flat planar and/or flexible objects.

Optionally, the item may be formed from paper, formed from plastics material, is a bank note, a passport, an ID card, a tax stamp and/or a legal document.

In accordance with a third aspect, there is provided a method of communicating with a plurality of capacitive coupled RFID tags, the method comprising the steps of: applying a radio frequency, RF, input signal to the plurality of capacitive coupled RFID tags; responding to the applied RF input by one of the plurality of capacitive coupled RFID tags, by varying its impedance, the varying impedance encoding a data signal; detecting a variation in the RF input signal caused by the varying impedance of the one of the plurality of capacitive coupled RFID tags, the variation encoding the data signal; decoding the data signal from the variation of the RF input signal; the capacitive coupled RFID tags of the plurality of capacitive coupled RFID tags that are not responding reducing their impedance while the one capacitive coupled RFID tags varies its impedance. Other methods of reading the above-mentioned CC-tags may be used Optionally, the variation in the RF input signal may be a variation in frequency, amplitude and/or phase.

Preferably, the method may further comprise the step of using an anti-collision protocol to determine which one of the plurality of capacitive coupled RFID tags responds to the radio frequency by varying its impedance.

Optionally, the plurality of capacitive coupled RFID tags may be stacked one above another or otherwise located in proximity to each other.

In accordance with a fourth aspect, there is provided a computer program comprising program instructions that, when executed on a computer cause the computer to perform the methods described above.

In accordance with a fifth aspect, there is provided a computer-readable medium carrying a computer program, as described above.

In accordance with a sixth aspect, there is provided a system comprising: any of the one or more capacitive coupled RFID tags described above; and a reader comprising an RF signal generator and decoder configured to decode the data signal.

The methods described above may be implemented as a computer program comprising program instructions to operate a computer. The computer program may be stored on a computer-readable medium.

The computer system (e.g. implemented within an integrated circuit) may include a processor or processors (e.g. local, virtual or cloud-based) such as a Central Processing unit (CPU), and/or a single or a collection of Graphics Processing Units (GPUs). The processor may execute logic in the form of a software program. The computer system may include a memory including volatile and non-volatile storage medium. A computer-readable medium may be included to store the logic or program instructions. The different parts of the system may be connected using a network (e.g. wireless networks and wired networks). The computer system may include one or more interfaces. The computer system may contain a suitable operating system such as UNIX, Windows (RTM) or Linux, for example.

It should be noted that any feature described above may be used with any particular aspect or embodiment of the invention.

Brief description of the Figures

The present invention may be put into practice in a number of ways and embodiments will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 shows a schematic diagram of a capacitive-coupled RFID tag being read by a reader; and Fig. 2 shows a schematic diagram of a plurality of capacitive-coupled RFID tags of figure 1 being read by the reader.

It should be noted that the figures are illustrated for simplicity and are not necessarily drawn to scale. Like features are provided with the same reference numerals.

Detailed description of the preferred embodiments

According to an example implementation, a capacitive-couple tag (CC-Tag) is a radio-frequency identification system (RFID) based on a single integrated circuit (IC). In normal applications, an integrated circuit is a piece of mono-crystalline silicon with a single side functionalized through different steps in order to integrate electronic components such as, but not limited to resistors, capacitors, diodes, transistors, etc. that are connected via metal layers.

Due to the small size of these components, electrical connectivity to the integrated circuit is provided using the presence of several "pads" or metallic layers large enough to be connected with small wires or probes. These "pads" are normally placed on the functionalized side of the integrated circuit.

The back side of the integrated IC is usually connected to the ground reference and presents no particular interest at electronic level. In the present CC-Tag, one large metallic pad is place on the functionalized side of the IC and the back side of the device provides a second pad. This structure allows the device to be freely positioned onto a surface without any need for a precise positioning or angular orientation and it the device may function regardless of which side the CC-tag is deposited onto a surface. This important feature simplifies the deposition process and reduces the related costs.

The IC may be formed using CMOS (complementary MOS) technology. Due to the presence of both n-type and p-type MOS on the same silicon substrate, a well-known effect known as "latch-up" may arise. This can be caused by a parasitic thyristor intrinsically existing in the CMOS structure, which can be triggered on by unexpected voltage spike and create a short-cut conduction between positive and negative contacts of a power supply of the device. This effect is normally reduced by placing extra ground contacts onto the substrate. This effect can be even stronger with CC-Tags due to the presence of a radio frequency (RF) signal coming from the back-side pad (i.e. semiconductor), which may be connected to a virtual ground of the IC. In order to avoid this effect, extra ground contacts may be placed in proximity of the MOS transistors.

In this example CC-Tag, the back-side of the integrated circuit (i.e. plain semiconductor substrate) is used as a potential electrical pad and is placed close to a second large pad on the top (opposite side) of the functionalized surface. In this example, the structure takes to the form of a thin plate (with a first and opposite surfaces). The system consists of an integrated circuit with one pad on the top and one on the bottom (or the other way round depending on any particular orientation), which can be used as capacitor plates from an ad-hoc developed reader.

The reader can communicate with the CC-Tag via a modulated electric field (Capacitive Coupling) without requiring electrical continuity between the reader and the tag.

Standard RFID tags have their input stage, composed of an inductor acting as an antenna and a tuning capacitor, tuned at the transmission frequency in order to optimize the signal transfer. CC-tags, on the contrary present only a capacitor at the input stage. Capacitors at high frequency present a short-cut behaviour; the cut-off frequency at which this behaviour is visible depends on the value of the capacitor itself and on the value of -10 -parasitic resistance of the circuit. This approach presents an important advantage, since the coupling capacitors are required to be just "high enough" in order to present a short-cut behaviour, without need for a precise tuning of the capacitor itself. This results in much lower dependence on the process parameters fluctuation in the deposition step.

For the same reason, CC-tags can even work at different frequencies at the same time, in general the higher the better. The solution shown in the figures (described in more detail below) offer several advantages: - The tag is extremely simple to manufacture, cheap and very robust. It can easily be fabricated along with standard integrated circuit technology.

- It does not require special alignment on the item to be tagged, which I turn reduces the complexity and cost of fabrication.

- The CC-Tag can also be read when in a stack, when it implements an anti-collision protocol for tags and reader.

CC-Tags can be thinned down below 50pm to result in an ultra-thin and flexible tag that can be enclosed in a variety of applications such as but not limited to banknotes, official and government documents, tax stamps, visas, holograms as well as any packaging of any goods, paper etc. Figure 1 shows a schematic diagram of a capacitive-coupled tag 10 being energised by and read by a tag reader 20. The tag reader 20 includes electrodes 30 that provide an electric field 70 by generating a radio frequency (RF) signal. This RF signal can be modulated and detected by the capacitive-coupled tag 10. As shown in this figure, the upper or top surface of the capacitive coupled tag 10 has an electrode 40, which is metallic.

The opposite or bottom surface of the capacitive-coupled tag 10 is left bare and forms a semiconductor electrode surface 50. Within or on the surface of a semiconductor substrate of the capacitive-coupled tag 10 is a circuit (i.e. an integrated circuit) 60 (not shown in detail in this figure). The electric field 70 is illustrated by arrows between the electrodes 30 of the reader 20. The metal may be aluminium, copper, gold, silver or a combination of separate metal layers (e.g. gold on aluminium). The IC may be fabricated upon or within the surface of the CC-tag 10 that does not have the metallic pad (i.e. the semiconductor pad surface). Preferably, the surfaces are parallel and planar.

Figure 2 shows a further schematic diagram of a plurality of capacitive-coupled tags 10, which are of the same form as shown in figure 1. Again, the reader 20 applies an electric field between its electrodes 30 to read each of the capacitive-coupled tags 10.

When there are more than one CC-tags 10 between the electrodes of the reader 30 then all but one of the CC-tags 10 responds to the incident RF input signal by modulating it (i.e. by altering its own impedance to provide a data signal). The remaining CC-tags reduce their impedance by for example, shorting their pads (metallic and semiconductor surfaces). This makes them electrically invisible to the reader. When the first CC-tag has provided its data signal it stops modulating the input RF signal and shorts its own pads.

Then another CC-tag responds to the input RF signal by modulating it as described above. The process continues until all CC-tags have provided their data. Therefore, the CC-tags can be stacked one above another without reducing the signal strength.

Separate CC-tags may communicate with each other to decide which one will respond to the signal. This may be achieved using an anti-collision algorithm, which can generate a random delay that results in a different time for the transmission of each tag. Alternatively, no communication is required and each CC-tag may respond to the input RF signal after a different (e.g. random) delay, which substantially reduces the risk of any two CC-tags responding at the same time (the actual transmission time may be short compared with the delay times).

Shorting the pads may be achieved by a transistor or transistors changing its status from "blocked" to "saturation" to act as an on/off switch, for example.

Further alternative implementations may be used. These may include any one or more of the following alternatives or advantages: * CC-Tags 10 can be embedded in ultra-thin structures or on their surface (such as paper sheets, banknotes, holograms, stamps, etc.), without requiring any antenna, alignment or specific positioning.

* CC-Tags 10 require no metallic contact with a reader. The Radio Frequency (RF) signal can go from the reader to the CC-tag 10 even through insulated layers providing these to be thin (less than a mm).

-12 - * A CC-Tag 10 may draw power from a reader when the reader is within reading distance. The CC-Tag 10 may respond by modulating the impedance between top and bottom electrodes (on/off switching).

* A CC-Tag 10 does not work on a circuit resonance and does not suffer of frequency detuning issues hence it is far more robust to production parameter fluctuations.

* CC-Tags 10 can be read whilst in a stack, provided they are aligned in the stack, or can be Capacitive-Coupled using an extra metallic layer placed onto the thin structure.

* CC-Tags 10 may be equipped with an anti-collision solution to avoid two or more tags communicating with the reader at the same time.

* When CC-Tags 10 are read in a stack, then they may follow the following procedure: Receive power supply and data from the reader for a certain time (charge time).

If a tag is supposed to answer, it can modulate its impedance to communicate with the reader.

If a tag is not supposed to answer, it can reduce its output impedance to a low level (switch on) to allow the reader signal to pass through to the tag that is targeted to answer.

When CC-Tags 10 are read whilst in a stack, they are electrically "in series" with each other. This means that the voltage across the stack is divided by the number of elements in the stack. The reader may provide an Adaptive Voltage in order to provide each CC-Tag 10 with enough voltage or power to operate. This may involve increase the applied electric field (e.g. voltage and/or power) to a level so that all of the CC-tags can be energised, for example.

As will be appreciated by the skilled person, details of the above embodiment may be varied without departing from the scope of the present invention, as defined by the appended claims.

For example, the CC-tags and readers may be operated using standard industrial, scientific and medical (ISM) radio band frequencies (i.e. the frequency of the input RF signal). CC-tags may operate at separate frequencies (i.e. to avoid collision of data signals). The reader may scan different frequencies, for example.

Many combinations, modifications, or alterations to the features of the above embodiments will be readily apparent to the skilled person and are intended to form part of -13 -the invention. Any of the features described specifically relating to one embodiment or example may be used in any other embodiment by making the appropriate changes.

Claims

-14 -CLAIMS: 1. A capacitive coupled radio frequency identification, RFID, tag comprising: a semiconductor substrate having a first planar surface and a second planar surface distal from the first planar surface; a metallic pad formed on the first planar surface of the semiconductor substrate; a circuit formed on the semiconductor substrate and electrically connected to the metallic pad and the second planar surface of the semiconductor substrate, the circuit configured to respond to a radio frequency, RF, input signal by providing a data signal encoded by varying an impedance between the metallic pad and the second planar surface of the semiconductor substrate.
2. The capacitive coupled RFID tag of claim 1, wherein the data signal is encoded by the varying impedance between the metallic pad and the second planar surface of the semiconductor substrate modulates the RF input signal.
3. The capacitive coupled RFID tag of claim 1 or claim 2, wherein RF input signal is provided by an external reader.
4. The capacitive coupled RFID tag according to any previous claim, wherein the circuit is further configured to be powered by the RF input signal.
5. The capacitive coupled RFID tag according to any previous claim, wherein the circuit is further configured to decode a signal encoded within the RF input signal and further wherein the data signal is provided in response to the decoded signal.
6. The capacitive coupled RFID tag according to any previous claim, wherein the circuit modulates the RF input signal by varying its frequency, amplitude and/or phase.
7. The capacitive coupled RFID tag according to any previous claim, wherein the circuit formed on the second planar surface of the semiconductor substrate.
-15 - 8. The capacitive coupled RFID tag according to any previous claim, wherein the circuit is configured to vary the electrical impedance between the metallic pad and the second planar surface of the semiconductor substrate by applying a short circuit between the metallic pad and the second planar surface of the semiconductor substrate.
9. The capacitive coupled RFID tag according to any previous claim, wherein a distance between an outside surface of the metallic pad and the second planar surface of the semiconductor substrate is equal to or less than 50pm.
10. The capacitive coupled RFID tag according to any previous claim, wherein the circuit is further configured to detect the presence of one or more further capacitive coupled RFID tags and in response, cease providing the data signal.
11. The capacitive coupled RFID tag of claim 10, wherein the circuit is configured to cease providing the data signal by applying a short circuit between the metallic pad and the second planar surface of the semiconductor substrate.
12. The capacitive coupled RFID tag of claim 10 or claim 11, wherein the circuit is configured to cease providing the data signal until the one or more further capacitive coupled RFID tags have provided their data signal.
13. The capacitive coupled RFID tag according to any of claims 10 to 12, wherein the circuit is further configured to cease providing the data signal according to an anti-collision protocol.
14. The capacitive coupled RFID tag of claim 13, wherein the anti-collision protocol is based on communications between the one or more capacitive coupled RFID tags: according to a pre-determined order of response, according to a negotiated response between the one or more capacitive coupled RFID tags, or a random number 30 generator.
15. The capacitive coupled RFID tag according to any previous claim, wherein the first planar surface is parallel with the second planar surface.
-16 - 16. The capacitive coupled RFID tag according to any previous claim, wherein the capacitive coupled RFID tag is flexible.
17. An item having the capacitive coupled RFID tag according to any previous claim embedded within it.
18. The item of claim 17, wherein a surface of the item is parallel with the first planar surface of the capacitive coupled RFID tag.
19. The item of claim 17 or claim 18, formed from paper, formed from plastics material, is a bank note, a passport, an ID card, a tax stamp and/or a legal document.
20. A method of communicating with a plurality of capacitive coupled RFID tags, the method comprising the steps of: applying a radio frequency, RF, input signal to the plurality of capacitive coupled RFID tags; responding to the applied RF input by one of the plurality of capacitive coupled RFID tags, by varying its impedance, the varying impedance encoding a data signal; detecting a variation in the RF input signal caused by the varying impedance of the one of the plurality of capacitive coupled RFID tags, the variation encoding the data signal; decoding the data signal from the variation of the RF input signal; the capacitive coupled RFID tags of the plurality of capacitive coupled RFID tags that are not responding reducing their impedance while the one capacitive coupled RFID tags varies its impedance.
21. The method of claim 20, wherein the variation in the RF input signal is a variation in frequency, amplitude and/or phase.
22. The method of claim 20 or claim 21 further comprising the step of using an anti-collision protocol to determine which one of the plurality of capacitive coupled RFID tags responds to the radio frequency by varying its impedance.
23. The method according to any of claims 20 to 22, wherein the plurality of capacitive coupled RFID tags are stacked one above another.
-17 - 24. A computer program comprising program instructions that, when executed on a computer cause the computer to perform the method of any of claims 20 to 23.
25. A computer-readable medium carrying a computer program according to claim 24.
26. A system comprising: one or more capacitive coupled RFID tag according to any of claims 1 to 17; and a reader comprising an RF signal generator and decoder configured to decode the data signal.