Card with metal layer and an antenna

In a smart card having an antenna structure and a metal layer, an insulator layer is formed between the antenna structure and the metal layer to compensate for the attenuation due to the metal layer. The thickness of the insulator layer affects the capacitive coupling between the antenna structure and the metal layer and is selected to have a value which optimizes the transmission/reception of signals between the card and a card reader.

BACKGROUND OF THE INVENTION

This invention relates to smart cards and in particular to multi layered smart cards having a metal foil/film/layer and an antenna structure for receiving or transmitting signals between the smart cards and a card reader.

As shown inFIGS. 1, 2(a),2(b) and2C a typical smart card includes an antenna structure coupled (directly or electromagnetically) to a microprocessor or microcomputer (also referred to as a “chip”) also located on or within the card. The antenna structure functions to enable contactless communication between a card reader (also referred to as a “transponder” or “transceiver”) and the microprocessor. That is: (a) signals emitted by the card reader are electromagnetically coupled via the antenna to the chip which receives and processes the signals; and (b) signals processed and emitted by the chip are electromagnetically transmitted via the antenna to the card reader. In the manufacture of certain smart cards it is highly desirable that a metal foil/film/layer be included among the card layers. However, the metal foil/film/layer presents a problem since it functions to attenuate (absorb) the signals transmitted between the card reader and the chip limiting communication or even making it impossible. As an example, it is a requirement that a card reader should be able to read a smart card located at a given distance from the card reader. Known cards with a metal foil/film/layer could not be read reliably at these established minimum required distances.

To overcome the attenuating effect of the metal foil/film/layer, an insulator layer may be formed between the metal film/foil/layer and the antenna structure. A conventional approach is to make the insulator layer very thick to decrease the attenuating effect of the metal foil/layer. However, this is not acceptable where the permissible thickness of the insulator layer is limited. As is known in the art there are numerous requirements which have to be met in the manufacture of cards. Some go to the structural integrity of the cards (e.g., they should not bend, delaminate) and be capable of use for several years and a large number of user cycles. So, the cards need to be formed using numerous layers with various requirements on the thickness and composition of the layers. Thus, it is not satisfactory to just make the insulator layer arbitrarily very thick since such thick layers interfere with other requirements in the manufacture of smart cards.

The problem of manufacturing a reliable smart card is even greater particularly when the card includes a metal layer which interferes with the transmission/reception of signals between the smart card and a card reader.

Thus, a significant problem faced by Applicants related to selecting the thickness of an insulator layer, extending within a prescribed range, which can provide reliable readings of card data by a card reader located at a prescribed distance and at a prescribed frequency of operation. An associated problem was finding a thickness for the insulator layer which provided improved transmission/reception.

SUMMARY OF THE INVENTION

Applicants invention resides, in part, in the recognition that the combination of an antenna structure and its associated electronics, including an RFID chip, formed on or within a card can be tuned by varying the distance (“d”) between the conductive wires forming the antenna structure and a metal film/foil/layer. “Tuning” as used herein includes enhancing the transmission/reception of signals between the card's antenna structure and a card reader (or like device), so that reliable communication can be had between the card and the card reader at predetermined distances and at predetermined frequencies. In accordance with the invention, “tuning” can be achieved by controlling the thickness of an insulating (non-conductive) layer or layers formed between the antenna structure and the metal film/foil/layer.

In fact, Applicants discovered that, in response to certain transmitted card reader signals, the amplitude of the signals received at the card's antenna were greater for some insulator thickness, which may be termed the preferred thickness (“Tp”), than for thicker insulator layers. Varying the thickness of the insulator layer, varies the distance “d” between the antenna structure and the metal layer and controls the capacitance between the antenna structure layer (23) and the metal layer (24). Varying the capacitance can be used to “tune” the structure to improve the read/write distance between a card reader and a card and the performance (reception/transmission) of the inductive coupling system.

The antenna or antennas can be formed on either side of the holographic metal foil/film/layer and the resulting finished card body can, for thin metal film/foil layers, be interrogated by a card reader from either side of the card body.

DETAILED DESCRIPTION

Referring toFIGS. 1 through 4there is shown a generally symmetrical type of card structure illustrating the use of buffer layers in a card construction to absorb the difference between layers having substantially different characteristics which enables many different types of sturdy and secure cards to be manufactured. Card5includes (in the Figures) a top section101a, a correspondingly symmetrically shaped bottom section101band a center section103.

Top section101aincludes a PVC overlay14amounted over a core PVC layer10a, overlying a buffer layer22a, which, in turn, overlies a PET layer12a, overlying a buffer layer25a.

Bottom section101bincludes a PVC overlay14bformed under a core PVC layer10b, underlying a buffer layer22b, which underlies a PET layer12b, underlying a buffer layer25b. The buffer layers reduce the stress between the very dissimilar materials enabling a more stable structure having a much greater life time and of greater sturdiness.

The center section103of card5formed between sections101aand101bincludes a metal layer24separated by an insulator layer21from the antenna structure layer23.

Metal layer24may be a holographic film or a metal foil. The metal layer may be used in the card for decorative purposes to give the card a metallic or rainbow coloring which reflects light in desirable ways. Or, to serve any other functional or cosmetic purpose. The metal film/foil layer24may be may be metalized or transparent, holographic or plain metalized non-holographic material. The thickness of the metal layer may range from about 1 micron to 200 microns.

The antenna structure layer23may include an antenna directly coupled to an RFID chip. Alternatively, it may include a booster antenna inductively coupled to a chip antenna which in turn is directly coupled to an RFID chip206. The antenna structure23may be part of an “RFID inlay” which includes an RFID chip and associated antenna(s) to communicate with a reader or like device. The antenna structure may be formed on or within a suitable plastic layer. The antenna structure23and its associated RFID chip206(which may be on same layer or on different layers) are intended to communicate wirelessly with a card reader100. The metal layer24interferes with the wireless communication as it attenuates (absorbs) the RF energy transmitted between the card reader and the card.

In cards embodying the invention, the insulator layer21is inserted between the antenna structure and the metal layer to counteract the attenuation due to the metal layer. The insulator layer may be formed of any material capable of insulating near field high frequency (HF) signals, but is typically PVC, PET, PETG, PC, latex, cellulose, fiberglass (Teslin), an adhesive and/or composites of these, or other polymers used in card construction. The thickness of the insulator layer21may typically range from 10 microns to 350 microns. The maximum thickness of the insulator layer is normally set by constrains pertaining to the various layers used to form the card. Within the permissible, or allowable, range of thicknesses there may be an optimum value of thickness which provides best results, as discussed below, and which, in accordance with invention, may be the selected value of insulator thickness. As discussed below, the thickness of the insulator may be selected to have a value which will optimize (tune) the transmission and reception of signals between a card reader100and the antenna structure or chip206.

In one embodiment the various layers of the rd ofFIG. 4were approximately as follows:(i) PVC laminate layers14a,14bwere 2 mils each,(ii) The core PVC layers10aand10bwere 6 mils each,(iii) The PET layers12aand12bwere 1 mils each, and(iv) The buffer layers22a,22b,25a, and25bwere 1 mil each(v) The antenna structure23and RFID chip layer were 2 mils(vi) The metal layer24was 2 mils; and(vii) The insulator layer was 3 mils.Note that depending upon properties desired and cost constraints polyester layers (the more expensive material) can be placed near both outside surfaces or in the center or unbalanced in the core such that a “bowed” card can be straightened after personalization with a clear lamination. In general, PVC is used to form the outer layer of a card because it enables the personalization of a card to be made more easily. PVC is also normally used because it enables thermal printing or embossing. PVC based materials are normally much cheaper than PET, whereby the greater use of PVC is desirable for economic reasons. Thus, the layers of PVC material are normally thicker (individually and in the aggregate) than the layers of PET and of buffer material.

Referring toFIGS. 1 through 4note that a composite card5may be formed using numerous layers of different materials which serve various purposes as described below. Pertinent to the present invention is the formation of an inductive coupling antenna structure23in close proximity to a holographic metal film/foil layer24. InFIGS. 1 and 4, the metal film24is shown located in or near the center of the card body. However this is by way of illustration only and the antenna structure and metal film could be located near the top or bottom of the card. The antenna structure23may be formed above a plastic layer or within a plastic layer. As shown inFIGS. 2A and 2Can RFID chip206may be directly connected to the antenna. Alternatively, as shown inFIG. 4, the RFD chip206may be located at a different level (layer) than the antenna structure and be inductively (or directly) coupled to the antenna structure. Thus, smart cards embodying the invention include an antenna either directly connected or inductively connected to an RFID chip to provide contactless communication with a card reader100. The term antenna structure23as used herein may refer to the antenna and/or to the antenna and its associated circuitry including the chip.

As shown inFIG. 4, a card reader or an RF transmitter100, can send signals to a smart card having an antenna structure23separated from a metal layer24by an insulator layer21. The antenna structure23is inductively coupled to an RFID chip206. As is known, a card reader (or transponder)100is used to interrogate the card by sending (transmitting) RF signals to the card which are electromagnetically coupled via the antenna structure to the RFID chip206. In response to the received interrogation signals, the card produces RF signals which are in turn transmitted to the card reader where they are received for processing. As noted above, a problem with having a metal layer24is that the metal layer attenuates (absorbs) the electromagnetic energy impeding the transmission/reception of signals between the card and the card reader.

The insulator layer21(which may be an adhesive layer or any suitable insulating layer as noted above) is interposed between the antenna structure23and metal layer24. An important aspect of the invention is the selection of the thickness “d” of layer21.

Applicants recognized that the antenna structure23and its associated electronics (e.g. RFID chip206) can be tuned by varying the distance (“d”) between the conductive wires forming the antenna structure and the holographic metal film which was done by controlling the thickness of layer21. Varying the distance “d” controls the capacitance between the conductive elements in layers23and24. This tuning can be used to improve the read/write distance and performance of the inductive coupling system. Varying the distance “d” can thus be described as seeking and finding a resonant or quasi-resonant frequency range.

Some insight into the interaction between the metal layer and the insulator layer and their effect on signal transmission/reception may be obtained by reference toFIG. 2D.

FIG. 2Ddepicts the modelling of the role of the foil and the insulator layer and represents an attempt to illustrate and explain the interactive effect of a metal foil and the role of an insulator layer, formed on a card, on the signal transmission between a card204(equivalent to card5inFIG. 4) and a card reader202(equivalent to card reader100inFIG. 4). The metal foil and the insulator layer may be represented by a network200which includes capacitors C4and C5and resistors R4and R5. The card reader may be represented by a transponder212and associated components R1, C1and R2and an antenna coil L1included in network202. The card antenna structure and associated electronics may be represented by antenna coil L2resistor R3capacitors C2and C3and an RFID chip206all included in a network204.

Resistors R4and R5change with foil (metal layer) thickness. The thicker the metal the lower the value of resistance. The values of C4and C5change as a function of the thickness “d” of the insulator layer between the metal layer and the antenna wires. The smaller (thinner) the insulator thickness the higher is the capacitance of C4and C5. Note that the one end of resistors R4and R5connected in common are shown retuned to ground (GND). This ground (GND) is a “virtual” ground in so far as it represents the grounding effect of the metal foil. That is, the metal foil absorbs the electromagnetic energy but it does not have an electrical path back to the chip or the transponder.

AlthoughFIG. 2Dprovides insight into the interaction of the metal foil and the insulator layer, it does not enable a user to determine a range of values of C4and C5which can optimize transmission/reception at a specified frequency and given metal thickness. It also does not enable a user to derive some form of an approximate equation yielding the resonant frequency.

In accordance with one aspect of the invention, a preferred set of values for the thickness of an insulator layer was determined for a selected structure of the type shown in the figures, as follows. A set of four cards was fabricated which varied only in that the thickness (T) of insulator layer21was varied in steps. The antenna layer23of the 4 sets of cards was the same and the metal layer of 50 microns was the same for the 4 sets. The insulator layer thickness of the 4 sets was varied to be 50, 100, 150 and 200 microns, respectively. Signals were transmitted from a transponder, such as RF transmitter100, and the amplitude of the signals received at the antenna structure was then measured for each card. As shown inFIG. 5, for signals emitted by the transponder100at a nominal carrier frequency of 13.56 MHz, the measured amplitude of the signals received at the antenna structure had: (a) a relative value of approximately 11 for an insulator thickness of 50 microns; (b) a relative value of approximately 38 for a thickness of 100 microns; (c) a relative value of approximately 80 for an insulator thickness of 150 microns; and (d) a relative value of approximately 62 for a thickness of 200 microns. This demonstrates that, for the particular structure shown in the figures, an insulator thickness in the range of 150 microns provides better reception than a thicker (i.e., 200 micron) insulator layer. This is a significant result demonstrating that testing can reveal a range of values for the thickness of the insulator layer which provide better results than would be expected based on the assumption that the thickest insulator provides the best results for transmission/reception of signals. Thus, as noted above, a quasi-resonant frequency range is located where improved transmission/reception is achieved.

For other card structures, a thickness “d” equal to 54 microns was found to be acceptable.

In accordance with the invention, card structures containing a set antenna structure and a set metal layer (of predetermined thickness) can be tested to ascertain the thickness of a separating insulator which will provide better or best reception between a card reader and the antenna structure.(a) This can be done on an individual card basis.(b) This method enables the manufacture of a few cards to determine best operating ranges and then follow through with the manufacture of batches of cards.(c) The method also enable a user to vary various parameters of the card (e.g., foil thickness and antenna structure) for a given insulator thicknesses and determine reliable operating ranges.(d) So, as used herein the “selected” or “preferred” insulator thickness (Tp) is defined as the insulator thickness Tp that gives the best (or at least acceptable) reception (at the antenna) to a transmitted signal from a card reader. This value of Tp will also apply for the signals emitted by the card chip via the antenna to the card reader.(e) Tp may be determined (found) for the frequency (e.g., 13.56 MHz) at which the card reader (or interrogating device) is operating to ensure best responses.(f) Tp may be determined empirically for a given metal configuration and its thickness and a given antenna structure configuration. At this time no satisfactory equation has been generated which can be used to accurately predict Tp for different configurations.(g) The value of Tp may be different if the antenna structure changes because the frequency response profile of the antenna will also change. The design of the antenna may have to change due to embossing requirements or other physical attributes of the cards.(h) The invention may be applicable to all types of metal cards. It is useful with “bulk” metal cards as well when it includes a ferrite layer. So it is relevant to all card constructions.(i) The invention is useful in making products in that for a given card product there are physical constraints which would dictate the antenna form factor and some of its surrounding layers. Based on those constrains, the insulating layer may be “tuned” as taught herein to maximize the signal response between the card and a card reader.(j) The invention is particularly useful since it teaches that given the constraints applicable to a card product, the insulator layer an be varied to optimize transmission/reception results. This goes beyond the prior art suggestion of just tuning the antenna.