TRANSPONDER TAGS

Abstract: A transponder tag for providing information to an interrogation station comprises a photo sensitive means (8, 14) for receiving light from an interrogation station, a circuit means (4) having data storage means (18) for storing data, and electrode means (6, 7) for capacitively coupling the tag to said station. The photo sensitive means (8, 14) is responsive to pulsed light from said station and is operable to energise said circuit (4) and to generate pulses which enable the reading of data from said data storage means (18) capacitively by the said station.

Description: TRANSPONDER TAGS The present invention relates to an electronic interrogation system which includes a transponder which can be attached to, or embedded in, an object for providing information relating to such object when interrogated by an interrogation station. Commonly such items are in the form of tags for external attachment and are known as transponder tags. Very commonly they operate by rf transmissions (e.g. microwave frequencies) from an interrogation station which serve both as a source of power for the transponder tag and as a means of triggering transmission of information from the tag. An important parameter of such tags is their operating distance, which may be from a few centimetres to metres. A problem with rf is the complexity required in the construction of the tag; in particular it requires an rf antenna, usually in the form of a helix, formed on the tag. It is also extremely well known to employ bar code readers for deriving information from objects to which bar codes are attached which operate by shining a beam of light onto the bar code from a close distance and by analysing the reflected light. Summary of the Invention It has been realised, in accordance with the invention there is a need for a very simple transponder tag having an operating range of millimetres and which can provide information automatically without requiring a swiping action. Such tags may, for example, be embedded in the surface of an object, for example a CD jewel case. It is an object of the present invention to provide a transponder tag item for providing information to an interrogation station which item is very simple and inexpensive in operation. The present invention as claimed in the attached claims provides tag for attachment to or embedding in an object and for providing information of such object when interrogated by an interrogation station which is simple and inexpensive. It will be appreciated that tags according to the invention may be manufactured inexpensively and may for example be based on a silicon die having a photosensitive area for powering the tag, and a metallised area for feeding back capacitively, information signals. As a result, no area is taken up by bond pads (which are normally necessary for tags based on an rf method of operation), and the silicon can be fabricated with a protective covering over the whole area of the die. Including the scribe lines, the die area might be 500 micrometres square, which would allow 125,000 unique tags to be manufactured from one 8" silicon wafer. Each manufactured die constitutes a tag, and may for example be embedded in the plastic surface of a finished product, for example the housing of an electrical item sold from a store or supermarket or a CD jewel case. The interrogation station, (tag reader) according to the invention, includes a light source which may be an infrared LED, to couple power to the tag, and preferably one or more capacitive plates to sense the voltage signal from the tag, the voltage being time varying in accordance with the information stored to represent a serial data stream. The information may be stored in the tag at the time of production, for example in ROM form wherein a metallisation mask is patterned to form the desired code. As preferred, the information is stored in a serial shift register, so that the information can be read out serially for energisation of the capacitive plate means, so that the plate provides a time varying voltage. Thus the code might be stored on the metalUsation mask, and used to select whether each stage of a shift register is set (to 1) or reset (to 0) on power up. Thus the information controls the initial state of the register. When the register is clocked, the bit pattern rotates around the register indefinitely. In a further embodiment, identification information can be written into the tag subsequent to production of the tag, the tag including suitable means for capturing the incident data from the light source. In a preferred embodiment, the light source of the interrogation station is pulsed or otherwise suitably modulated to provide timing pulses to the tag in order to synchronise operation of the tag with that of the station, so that data transmitted from the tag is accurately synchronised to the station. In addition, the station may provide with the timing pulses information for storage or for initiating certain operations of the tag. The preferred light excitation of the tag is at infra-red wavelengths, where silicon is most sensitive. As a result, the tag may be invisibly embedded below the surface of a product. Its covering should be at least partially transmissive to infrared, but may for example be overprinted. Thus the present invention places no restrictions on the appearance of the product it protects. Since the tag may be very small in dimensions, the product may have a degree of flexibility. Since as preferred the tag comprises a silicon die with no bond wires or exposed aluminium, it will be much better protected from corrosion and moisture than conventional silicon die. Brief Description of the Drawings A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure lis a schematic plan view of a tag according to the invention, comprising a silicon die; Figure 2 is a circuit diagram of the circuit of the tag of Figure 1 ; Figure 3 is a circuit diagram of the circuit of a tag reader; and Figure 4 is a circuit diagram of a tag constructed in accordance with a second embodiment of the invention. Description of the Preferred Embodiment Referring now to Figure 1, in a preferred embodiment, a tag comprises a silicon substrate 2 on an upper surface of which is formed circuitry 4 by CMOS processes. Circuitry 4 includes relatively large rectangular capacitive plates 6, 7 positioned on either side of a row of three photosensitive strips 8. For a die whose dimensions are 500 microns on a side, there is 100 microns free around each edge to allow for damage. That leaves an area of 300 microns square in the middle. Each capacitive plate 6,7 is 300 by 100 microns, leaving a third strip of 300 by 100 microns for the photo sensors 8. Most of the circuitry described below is constructed under the plates. If the circuitry requires top level metallisation - as used by the plates - then some thin lines of metal may need to be separated from the plates and used for circuitry interconnections. Since plate 7 can be connected to the silicon substrate as a ground reference, and as unmetalUsed silicon is conductive, plate 7 may be effectively formed by the unmetallised silicon area and this need not, in practice, be covered with metal. Referring now to Figure 2, the internal CMOS circuitry of the tag is shown as comprising the photo sensors 8 connected in series and providing potentials Vγ)T) on supply rail 10 and Vgs on supply rail 12 when irradiated. A photo sensitive diode 14 connected in series with a resistor 15 across the supply rails, senses the light irradiation, and as a result produces a current, which in turn develops a voltage across the resistor 15. An amplifier 16 which may be a standard digital inverter, detects the voltage across resistor 15 and is operative to speed up the voltage transition at its input, and give a clean clock edge. Amplifier 16 provides an output clock signal CK in response to pulsed light irradiation. The clock CK is coupled to the clock input of a serial data register 18 for shifting information data held in the register, one stage per clock pulse. The data may be 64 bits in length, although longer sequences may be employed, particularly if the data is stored and transmitted in encrypted form. An output stage of the shift register 18 provides a data Q output on an output line 20 to one (6) of a pair of capacitively coupled plates 6,7, and is fed back to an input stage of the register on Une 22, whereby to recycle continually the data through the register 18. The data held in the register 18 is normally programmed therein at the time of formation of the CMOS circuitry 4. Capacitive plate 7 is coupled to supply rail 12. An alternative is to have two plates 6, 7, and connect one to the signal data stream, and the second to the same data signal passed through an inverter (as shown in Figure 3). This will give approximately twice the level of received signal at the interrogation station. Referring to Figure 3, circuitry for a tag reader comprises capacitive plates 30, 32, each 1 cm square positioned side by side for sensing the voltage variations by capacitive coupling when plates 6, 7 of the tag pass close by the plates 30, 32. Plates 30, 32 are connected to the inverting and non inverting inputs of amplifier 34, whose output is coupled to a synchronous decoder 36, which provides an output data signal representing the information provided by the tag. The tag reader in addition comprises an infrared LED light source 38 which is periodically energised by timing generator 40 in order to transmit a stream of timing Ught pulses to the tag. Whilst the data rate of the stream may be as high as 100 kbits per second, in the present embodiment the light is pulsed off for 1 microsecond every 100 microseconds, giving a data rate of 10 kbits/second. Manchester coding is preferred for robustness, giving an overall data rate of 5 kbits/second. Timing generator 40 is also coupled to synchronous decoder 40 in order to provide timing slots in which incoming data from plates 30, 32 is analysed. In the embodiment described above the transponder tag is essentially a "read only" tag. There are instances where it would be useful to be able to write data to the transponder tag as well. For example, it would be possible to manufacture sets of blank tags which are uniquely initiated by optically writing data to the tag. The tag may include a WORM (Write Once Read Many times) data store. Referring to Figure 4 there is shown a schematic diagram of a tag which has a write capacity. In Figure 3 components similar to those shown in Figure 2 are given the same reference number. In greater detatt, the internal circuitry 4 of the tag of Figure 4 comprises the photo sensors 8 connected in series and providing potentials VDD on supply rail 10 and Vss on supply rail 12 when irradiated with Ught. A resistor R10 and capacitor CIO are connected in series across the supply rails 10, 12 a transistor TRIO is connected in parallel across the capacitor CIO. As in Figure 2, the input of the operational amplifier 16 is connected at the junction of the photo sensitive diode 14 and resistor 15 and provides an output clock pulse CK to the clock input of a shift register of the CMOS logic circuits 4. A second operational amplifier 25 is connected between the junction of the diode 14 and resistor 15 to provide data input signals to the data input of the CMOS logic circuits 4. The output Q and inverted output Q are supplied respectively to the plates 6 and 7. The components which give the write capability are RIO, CIO and TRIO together with modifications to the CMOS logic circuits shown in Figure 4. The photo sensors 8 and diode 14 are irradiated with pulsed light, and on each clockpulse CK generated by the ampUfier 16, the data input of the CMOS logic block is sampled. Immediately the reset output is pulsed high by the transistor TRIO and the capacitor CIO discharges. When the next clock pulse CK is generated, the state of the data input Une to the CMOS circuits will depend upon how long capacitor CIO has had to charge up. If the clock pulses are close together, the voltage across the capacitor CIO will be low; if the clock pulses are far apart the voltage across CIO will be high. In this way, the irradiation of the sensors 8 and diode 14 with a first sequence of light pulses (which ideally is in the infra red spectrum) effectively generates a clock pulse which operates the shift register 18 and coded data is input in to the shift register. Thereafter when it is required to read the data from the shift register 18 the irradiation of sensors 8 and diode 14 with pulsed light generates clock pulses to shift the data in the register and generate a voltage at plates 6 and 7. Clearly, the circuitry of the CMOS logic circuits may include data storage and means to check that the input data is not corrupt. It may also include means, such as a Non Volatile Random Access Memory (NVRAM) or WORM (Write Once Read Many times) storage so that subsequent unauthorised writing of data is prevented.

Claims: CLAIMS 1. A transponder tag for providing information to an interrogation station, said tag comprising a photo sensitive means for receiving light fromA an interrogation station, a circuit means having data storage means for storing data, and electrode means for capacitively coupling the tag to said station, characterised in that the photo sensitive mans (8, 14) is responsive to pulsed light from said station and is operable to energise said circuit and to generate pulses which enable the reading of data from said data storage means capacitively by the said station. 2. A tag according to claim 1 further characterised in that the photo sensitive means (8, 14) and said circuit (RIO, CIO, TRIO) is operable to generate pulses which enable data to be written optically to said data storage means. 3. A tag according to claim 1 or 2 wherein the tag is formed as a silicon die. 4. A tag according to claim 3, wherein the die comprises a substrate having on a surface thereof a plurality of photosensitive strips. 5. A tag according to claim 3 or 4, wherein the die comprises a substrate having on a surface thereof one or more plates for capacitive coupling with an interrogation station. 6. A tag according to any preceding claim, wherein the data storage means includes a serial shift register for storing data as a sequence of bits, and means for reading out the bits sequentially to said one or more plate. 7. A tag according to any preceding claim, wherein the light beam is modulated at a predetermined rate, and the tag includes means for detecting the light modulation and synchronising operation of the circuit means therewith. 8. A tag according to any preceding claim, wherein the tag includes means for receiving and storing information provided via the light beam of the interrogation station. 9. A tag according to claim 1 or a system according to claim 2, substantially as described with reference to the accompanying drawings. 10. A transponder system comprising a tag as claimed in any one of claims 1 to 9, and an interrogation station having means to generate pulsed Ught, and electrode means for capacitively coupling the station to the electrodes of the tag.

Inventors: GREEN IAN MACDONALD, GREEN, IAN, MACDONALD
Applicants: CENTRAL RESEARCH LAB LTD, GREEN IAN MACDONALD, CENTRAL RESEARCH LABORATORIES LIMITED, GREEN, IAN, MACDONALD
Application year: 1997
Main classification: G06K7
IPC codes: G06K7, G06K17, G06K19, H01L27, H01L31