In a contactless chip card, also referred to as a Smart Card, or in a so-called dual interface chip card, a communication between a chip and a card reader may be established contactlessly (in a case of the dual interface chip card in addition to an exposed chip contact that is configured to be physically contacted by the card reader). An interface for the contactless communication may include a chip antenna, which may be disposed on or in the chip card and may be in contact with the chip. The chip antenna and the chip may both be disposed on a chip card module. Such a combined arrangement of the antenna, which may be formed as a coil, and the chip on the chip card module may be referred to as coil-on-module (CoM).
For improving a performance of the contactless interface, in particular for increasing a distance up to which a contactless communication between the chip and the chip card reader may be possible, an amplifier antenna, also referred to as booster antenna, may be disposed on or in the chip card. The booster antenna may include a wire that may be arranged as a coil. The booster antenna may be inductively coupled to the chip antenna and thereby to the chip (or, more generally, to a semiconductor device). There may not be a galvanic interconnect between the module and the booster antenna.
Such Smart Cards may for example be used for transportation, banking, and government ID applications, and may typically work at an operating frequency of 13.56 MHz.
The wire booster antenna for such a Smart Card may typically be formed by embedding the wire in a sheet of plastic material (e.g. PVC, PC, PET-G), which may also be referred to as a chip card substrate, thereby forming an antenna sheet. The antenna sheet may be laminated with additional plastic sheets to a final thickness of the Smart Card (typically 0.80 mm).
The Smart Card, e.g. a resonance frequency of the boost antenna, may be tuned to a desired value. The resonance frequency may depend on various properties of the antenna, e.g. on a resistance, a capacitance, and an inductance. Thus, these properties may be set to defined values. These properties can be influenced by a geometry, which may include an arrangement, of the wire antenna.
As shown in FIG. 1A, structures with mainly inductive properties may be created by embedding wires 108, 108i, 1081 in parallel coil lines with similar orientation. “Similar orientation” may be understood to mean that, for the process of laying the parallel coil lines, a direction of laying progress, also referred to as winding direction, is the same for the parallel coil lines with the mainly inductive properties. Another way to describe this property may be a helicity, which may be the same for the parallel coil lines with the mainly inductive properties. An additional indication of “i” in the reference 108i for the wire 108 refers to the mainly inductive property of a specified portion of the wire (in FIG. 1A the complete wire, because all of it may have mainly inductive properties), and an additional index of “1” in the reference 1081 for the wire 108 refers to the direction of laying progress (the orientation) of a specified portion of the wire (in FIG. 1A the complete wire, because all of it may have the same direction of laying progress, which in the shown example may be counter-clockwise, as indicated by arrows).
As shown in FIG. 1B, structures with mainly capacitive properties (indicated by the “c” of the wire reference 108c) may be created by embedding wires 108c, 1081, 1082 in parallel coil lines with alternating orientation 1 and 2. “Alternating orientation” may be understood to mean that, for the process of laying the parallel coil lines, a direction of laying progress is opposite for the parallel coil lines with the mainly inductive properties. Another way to describe this property may be a helicity, which may be opposite for the parallel coil lines with the mainly conductive properties. In the shown example, the “1”-direction of laying progress (the orientation) may be counter-clockwise, and the “2”-direction of laying progress (the orientation) may be clockwise, as indicated by arrows.
To create such coils with alternating orientation, typically the first orientation, i.e. the wire portion 1081 with the first orientation, is embedded first (see FIG. 2A), afterwards the alternating orientation (with the opposite helicity), i.e. the wire portion 1082 with the second orientation, is embedded in between the wire portions of the first orientation (see FIG. 2B, crosses in the wires of FIG. 2A and FIG. 2B are indicative of the first orientation, circles in the wires of FIG. 2B are indicative of the second orientation). The capacitance C may be influenced e.g. by a length l of the parallel wires 108c, by a diameter 2R of the wires 108c and by a distance d of the wires: C=πεl/arcosh(d/2R) (see FIG. 3 for a graphic representation of the variables and of the arcosh function; ε may be a permittivity of a medium between the wires).
The resistance of the booster (wire) antenna 108 may be influenced e.g. by an overall length of the wire, by a diameter of the wire (smaller diameter=higher resistance) and by a specific resistance of the wire (e.g. pure Cu wire has a lower resistance than CuNi alloys).