Patent Description:
RFID uses magnetic, electric, or electromagnetic fields transmitted by a reader system to identify itself and, in some instances, provide additionally stored data. RFID tags typically include a semiconductor device commonly called the "chip" on which are formed a memory and operating circuitry, which is connected to an antenna. Typically, RFID tags act as transponders, providing information stored in the chip memory in response to a radio frequency ("RF") interrogation signal received from a reader, also referred to as an interrogator. In the case of passive RFID devices, the energy of the interrogation signal may also provide the necessary energy to operate the RFID device.

RFID tags are generally formed by connecting an RFID chip to some form of antenna. Antenna types are very diverse, as are the methods used to construct the same. One construction method of making RFID tags is to use a strap, which is a relatively small device with an RFID chip connected to two or more conductors that can be coupled to an antenna. Said coupling can be achieved using a conductive connection, an electric field connection, magnetic connection or a combination of coupling methods. Another method known in the art is direct chip attachment in which the chip is directly attached to the antenna without the utilization of any sort of strap or other device to aid in the connection of the chip to the antenna.

RFID tags may be incorporated into or attached to articles to be tracked. In some cases, the tag may be attached to the outside of an article with adhesive, tape, or other means and in other cases, the tag may be inserted within the article, such as being included in the packaging, located within the container of the article, or sewn into a garment. The RFID tags may be manufactured with a unique identification number which is typically a simple serial number of a few bytes with a check digit attached. This identification number is incorporated into the tag during manufacture. The user cannot alter this serial/identification number and manufacturers guarantee that each serial number is used only once. Such read-only RFID tags typically are permanently attached to an article to be tracked and, once attached, the serial number of the tag is associated with its host article in a computer data base.

A number of RFID antenna types require a resonant element as part of the overall structure. The resonant element is typically the combination of an inductor formed as part of the antenna and the capacitance of the RFID chip, and is capable of performing a number of different functions. For example, the resonant element may be part of a network matching the impedance of the chip and antenna for optimum power transfer, or coupling magnetically to a reader system at or near the resonant frequency. <CIT> discloses RFID tags provided for incorporation into the packaging of a microwavable food item, with the RFID tags being configured to be safely microwaved. The RFID tags include an antenna defining a gap and is configured to operate at a first frequency. The RFID tags further include a shield structure configured to limit a voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency. <CIT> discloses a method for efficiently producing a plurality of EAS or RFID tags or inlays that form a label ready for use. The process utilizes a first web of RFID chip straps or capacitor straps that are releasably secured to a liner using only a low tack adhesive and utilizes a second web of coils or antennas which are secured to a second liner. <CIT> discloses an RFID tag including: a tabular dielectric veneer; an antenna pattern that extends over a top surface and an undersurface of the dielectric and forms a loop antenna having both ends existing on one surface of the top surface and the undersurface; a circuit chip that is electrically connected, to the antenna pattern; a first electrode that is connected directly or via a conductor to one end of the both ends of the loop antenna and spreads on the one surface of the top surface and the undersurface; and a second electrode that is connected via a conductor to the other end of the both ends of the loop antenna and spreads on the other surface of the top surface and the undersurface, along the first electrode.

Unfortunately, a current limitation of present day RFID tag designs is achieving the desired resonance inside a relatively small area. The chip capacitance must be combined with an inductor according to the known resonant frequency formula: Fres=<NUM>/(2πSQRT(LC)), where L is the induction in henrys and is related to the length of wire or flat strip of conductor and its diameter/width, F equals the frequency in hertz and C is the capacitance in farads.

To achieve a given inductance, a certain amount of length and width must be accommodated as part of the RFID antenna to resonate the chip capacitance. Making a line narrow requires tighter manufacturing tolerances and increased resistance, which increases the amount of energy loss in the structure and, therefore, reduces the efficiency of the RFID tag and its operational range. In RFID antennas it is common to fold-up the inductor to fit inside an area in which the two ends are connected to the RFID strap/chip.

Consequently, it would be advantageous to have a method of adding capacitance across the capacitance of the attached RFID strap to reduce the amount of inductance needed to resonate at the desired frequency. The present invention discloses a method of adding said capacitance by using shielded straps with RFID tag designs. Specifically, the RFID strap device comprises a bridge conductor which couples the antenna and pair of strap pads together. This coupling between conductors increases the total capacitance of the RFID strap device. Further, the presence of the bridge conductor also reduces the area occupied for a given inductance, and provides a higher effective capacitance when the bridge strap is connected to the antenna.

The following presents a simplified summary in order to provide a basic understanding of some aspects of the disclosed innovation. This summary is not an extensive overview, and it is not intended to identify key/critical elements or to delineate the scope thereof. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. The scope of the claimed invention is defined by the appended claims.

The subject matter disclosed, in one aspect thereof, comprises a method of increasing capacitance by incorporating a second bridge conductor into an RFID strap device. More specifically, the RFID strap device comprises a bridge (or second) conductor coupled to a strap (or first) conductor via a separating dielectric. The RFID strap device is also coupled to a separate antenna conductor on a base substrate. The antenna can be made of aluminum foil, and the base substrate is typically a paper. Further, the second bridge conductor, the first strap conductor, and the antenna conductor overlap each other to provide a mutual area with separating dielectrics, and creating a plurality of capacitors.

In another example, the area of the bridge conductor is modified via a cutting process to alter the bridging capacitance. For example, if the area of the bridge conductor is larger than the area of the pair of strap pads, high bridging capacitance is provided. If the area of the bridge conductor is smaller than the area of the pair of strap pads, low bridging capacitance is provided.

According to some embodiments of the present disclosure, a radio-frequency identification (RFID) device comprises a first strap conductor comprises at least one pair of strap pads and an RFID chip connected to the at least one pair of strap pads, a second conductor as a bridge conductor, and a dielectric positioned between the first strap conductor and the second conductor, wherein the second conductor, the first strap conductor, and an antenna conductor are arranged to overlap each other to provide a mutual area with separating dielectrics such that the second conductor, the first strap conductor, and the antenna conductor are coupled via a plurality of capacitors having a capacitance value, and wherein, a capacitor formed by the first strap conductor, and the antenna conductor has a larger capacitive value to capacitors formed by: the coupling between the second conductor and the first strap conductor, and the coupling between the second conductor and the antenna conductor.

In some examples, the at least one pair of strap pads is coupled to the antenna conductor via a conductive adhesive. In embodiments, the at least one pair of strap pads is coupled to the antenna conductor via capacitance. In some embodiments, the antenna conductor is attached to a base substrate.

In some embodiments, the second conductor is a bridge. In some embodiments, the second conductor, the first strap conductor, and the antenna conductor overlap each other to provide a mutual area with separating dielectrics and a plurality of capacitors having a value. In further examples, the value of the plurality of capacitors is determined by (i) the mutual area, (ii) a dielectric constant of a material between the antenna conductor, the first strap conductor and the second conductor, and (iii) an amount of distance separating the antenna conductor, the first strap conductor and the second conductor.

In some examples, an area of the second conductor is larger than an area of the pair of strap pads. In other examples, an area of the second conductor is smaller than an area of the at least one pair of strap pads.

In some examples, the second conductor is modified via a cutting process. Also disclosed, a shape and an area of the second conductor is modified via a cutting process. In some examples, the cutting process is a laser cut line.

In some aspects of the present invention, a radio-frequency identification (RFID) strap device comprises a first strap conductor comprised of a pair of strap pads and an RFID chip connected between the pair of strap pads, a second conductor as a bridge conductor, and a dielectric positioned between the first strap conductor and the second conductor, wherein the second conductor, the first strap conductor, and an antenna conductor are arranged to overlap each other to provide a mutual area with separating dielectrics such that the second conductor, the first strap conductor, and the antenna conductor are coupled via and a plurality of capacitors having a capacitance value, and wherein, a capacitor formed by the first strap conductor, and the antenna conductor has a large capacitive value to capacitors formed by: the coupling between the second conductor and the first strap conductor, and the coupling between the second conductor and the antenna conductor.

In some examples, the value of the plurality of capacitors is determined by the (i) mutual area, (ii) a dielectric constant of a material between the antenna conductor, the first strap conductor, and the second bridge conductor, and (iii) an amount of distance separating the antenna conductor, the first strap conductor and the second bridge conductor.

In some examples, an area of the bridge conductor is larger than an area of the pair of strap pads. In examples, an area of the bridge conductor is smaller than an area of the pair of strap pads. In some examples, the second bridge conductor is modified via a cutting process.

The present disclosure also contemplates a method of making shielded straps having an increased capacitance across a RFID device, comprising providing a bridge conductor, a strap conductor comprising a pair of strap pads, a dielectric positioned between the strap conductor and the bridge conductor, and an antenna conductor, attaching the antenna conductor to the pair of strap pads of the strap conductor, and attaching the bridge conductor to the pair of strap pads of the strap conductor, wherein the bridge conductor, the strap conductor, and the antenna conductor are arranged to overlap each other to provide a mutual area with separating dielectrics such that the bridge conductor, the strap conductor, and the antenna conductor are coupled via a plurality of capacitors having a capacitance value, wherein, a capacitor formed by the strap conductor, and the antenna conductor has large capacitive value to capacitors formed by: the coupling between the bridge conductor and the strap conductor, and the coupling between the bridge conductor and the antenna conductor.

In some examples, a method of making shielded straps having an increased capacitance across a RFID device further comprises modifying one or more of a shape and an area of the bridge conductor via a cutting process. In some examples, the cutting process is performed before attaching the antenna conductor to the pair of strap pads. In other examples, the cutting process is performed after attaching the antenna conductor to the pair of strap pads.

To the accomplishment of the foregoing and related ends, certain illustrative aspects of the disclosed innovation are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles disclosed herein can be employed and is intended to include all such aspects and their equivalents. Other advantages and novel features will become apparent from the following detailed description when considered in conjunction with the drawings.

The present invention discloses a method of using shielded straps with RFID tag designs. Specifically, the RFID strap device comprises a bridge conductor which couples the antenna and at least one pair of pads (also referred to herein as strap pads or strap conductors) together. Thus, the coupling between the bridge conductor and the strap conductor, the coupling between the bridge conductor and the antenna conductor, and the coupling between the antenna conductor and the strap conductor increases the total capacitance of the RFID strap device.

The amount the capacitance increases depends on one or more of (i) the overlap area between (a) the bridge conductor and the strap conductor, (b) the bridge conductor and the antenna conductor, and (c) the antenna conductor and the strap conductor; the dielectric constant and thickness of intervening materials between each of the strap conductor, the bridge conductor, and the antenna conductor. In most applications an increase of up to <NUM> times the capacitance of the chip attached to a strap without a shield would be desirable, as higher values would make design of a broad-band antenna for an RFID tag coupled to the strap difficult. For example, a typical strap used for a UHF RFID tag may have a capacitance in the region of 1pF, therefore a shielded strap would have a range of capacitance between 1pF and 4pF.

The increased capacitance provided by the presence of the bridge conductor can have a number of beneficial effects on the design of the RFID tag it is used with. For example, having an increased strap capacitance reduces the required inductance to achieve resonance, as discussed further herein.

It is common for a UHF RFID tag to include an inductive element as part of the antenna connected across the strap, the inductive element intended to resonate at a given frequency, for example, the intended operating frequency of the RFID tag. This inductor is generally made as a planar loop of a given width of conductor and area. Because having an increased strap capacitance, which can be achieved by use of a bridge conductor, reduces the required inductance to achieve resonance, a designer can make a loop having a smaller area, and hence occupy less of the total area available for the rest of the antenna structure, thus allowing increased performance to be achieved. Alternatively, use of a loop having a smaller area can allow the use of a wider conductor. A wider conductor can have a number of benefits; for example: the resistance is lower, and hence less energy is lost when a current flows through it at the operating frequency; a different fabrication method can be used, for example, an etching process is required to define a <NUM> line, whereas a cutting process may be used for a <NUM> line, a cutting process advantageously being lower cost than an etching process.

Referring initially to the drawings, <FIG> illustrate an RFID device <NUM> that incorporates at least a second conductor <NUM>. The second conductor may be, for instance, but is not limited to, a bridge or shield conductor. While the present disclosure discusses the utilization of a second conductor <NUM>, the present disclosure further contemplates the utilization of any number of additional conductors and is not limited to a specific amount. Specifically, the RFID device <NUM> comprises a first conductor, which is at least one pair of conductor pads <NUM> and a second conductor <NUM> (also referred to as a bridge or shield conductor) with a dielectric <NUM> positioned between the second conductor <NUM> and the at least one pair of conductor pads <NUM>. In some embodiments, the first conductor can be a strap.

The second conductor <NUM> can be any suitable conductor as is known in the art, such as, but not limited to, an aluminum foil, a copper foil or a printed conductive ink. Further, the second conductor <NUM> can be any suitable size, shape, and configuration as is known in the art without affecting the overall concept of the invention. One of ordinary skill in the art will appreciate that the shape and size of the second conductor <NUM> as shown in <FIG> is for illustrative purposes only and many other shapes and sizes of the second conductor <NUM> are well within the scope of the present disclosure. Although dimensions of the second conductor <NUM> (i.e., length, width, and height) are important design parameters for good performance, the second conductor <NUM> may be any shape or size that ensures optimal performance during use.

The RFID device <NUM> further comprises an RFID chip <NUM> that is positioned between the conductor pads <NUM>, and is mounted on a suitable carrier (shown in <FIG>), such as a plastic, paper, fabric, corrugated cardstock, foam or any other suitable material. The second conductor <NUM> is then added to the other or opposite side of the strap dielectric <NUM> from conductive pads <NUM> and RFID chip <NUM>, coupling to the pair of conductor pads <NUM> and potentially to a separate antenna conductor.

As shown in <FIG>, the RFID device <NUM> comprises a bridge conductor <NUM> (also referred to as a second or shield conductor) connected to, or in communication with, an antenna conductor <NUM>. Specifically, the bridge conductor <NUM> is coupled to a conductor such as a strap conductor <NUM> and comprises a dielectric <NUM> positioned between the bridge conductor <NUM> and the strap conductor <NUM>. The strap conductor <NUM> further comprises at least one pair of strap pads <NUM> with an RFID chip <NUM> positioned between the strap pads <NUM>. The strap pads <NUM> may then be attached to the antenna conductor <NUM> via any suitable method as is known in the art, such as the application of a conductive adhesive or non-conductive adhesive (not shown). When attached to the antenna conductor <NUM> by a conductive adhesive, the strap pads <NUM> may be coupled to the antenna conductor via the conductive adhesive. In other embodiments, the coupling between the antenna conductor <NUM> and the strap pads <NUM> is via capacitance or any other suitable method of coupling as is known in the art, such as magnetic coupling. For instance, a magnetic loop can couple to an antenna, such as antenna conductor <NUM>, when it is adjacent to it. Additionally, the antenna conductor <NUM> can be made of any suitable material that is known in the art, such as, but not limited to, aluminum foil, copper foil, or printed conductive ink. The antenna conductor <NUM> can then be attached to an antenna base layer <NUM> to complete the RFID strap device <NUM>. In another embodiment shown in <FIG>, an RFID device <NUM> utilizing a strap comprises at least three conductive layers: a bridge layer <NUM>; a pair of strap pads <NUM>, to which RFID chip <NUM> is attached; and an antenna conductor <NUM>. While <FIG> illustrates the utilization of three conductive layers, the present invention is not limited to any number of conductive layers. These three conductive layers (<NUM>, <NUM>, and <NUM>) overlap each other to provide a given mutual area <NUM> with separating dielectrics, thus creating capacitors. The value of each capacitor is determined, at least in part, by the mutual area <NUM>, separating distance, and dielectric constant of the material between each conductive layer (<NUM>, <NUM>, and <NUM>). Additionally, there are small fringing capacitors created on the RFID device <NUM>, but generally these are smaller than the overlap capacitances created by the conductive layers (<NUM>, <NUM>, and <NUM>).

Additionally, <FIG> discloses a RFID device <NUM> and the coupling between a strap conductor <NUM>, an antenna conductor <NUM>, and a bridge conductor <NUM>. As disclosed, the coupling is not via the conducting dielectric <NUM>, but rather via capacitance. CSA represents the coupling <NUM> between the strap conductor <NUM> (i.e., strap pads) and the antenna conductor <NUM>. In one embodiment, a thin dielectric adhesive is used for this joint, making this capacitance large. CBS represents the coupling <NUM> between the bridge conductor <NUM> and the strap conductor <NUM>, and CBA represents the coupling <NUM> between the bridge conductor <NUM> and the antenna conductor <NUM>. CC is the capacitance of the RFID chip <NUM>. Generally, and considering that CSA is relatively large in comparison to the other capacitances, CBS and CBA are effectively in parallel; therefore, the capacitance added in parallel across CC can be expressed by the following formula: (<NUM>/CB) = <NUM>((CBA + CBS)), where CB is the total capacitance added by the presence of the bridge conductor <NUM>, and the capacitance presented to the antenna conductor <NUM> and an inductor structure as part of the resonating element is increased to CC + CB. Alternatively, the joint may be conductive by using an isotropic conductive paste, an anisotropic conductive paste, thermal, laser or ultrasonic welding, crimping or other methods.

<FIG> discloses one aspect of the invention of the present disclosure, specifically, a loop inductor <NUM> with a standard strap. The loop inductor <NUM> is shown at the typical inductor size to provide the desired resonant frequency, with CC being the capacitance of the RFID chip <NUM>. In contrast, <FIG> discloses a loop inductor <NUM> with a bridge conductor, and thus having a higher capacitance. For example, in one embodiment, the capacitance of the loop inductor is in the region of 1pf to 4pF at UHF frequencies. Because the bridge conductor increases the capacitance of the loop inductor, it also advantageously reduces the inductor size needed to provide the desired resonant frequency, with CC being the capacitance of the RFID chip <NUM> and CB being the total capacitance <NUM> added by the presence of the bridge conductor. Thus, the area occupied for a given inductance, assuming that the line width is unchanged, is reduced by the presence of the bridge conductor and the higher effective capacitance given when the bridge conductor is connected to the antenna.

As shown in <FIG>, a loop area or central matching resonator <NUM> without a bridge conductor is shown. Specifically, the folded dipole length <NUM> is shown within the given area <NUM> with the standard strap. <FIG> discloses a loop area or central matching resonator <NUM> with a bridge conductor. Specifically, the folded dipole length <NUM> is shown within the given area <NUM> with the bridge conductor. Thus, the benefit of using the bridge conductor on an antenna that is required to fit inside a given area <NUM> is shown with an increase in folded dipole length <NUM>. Specifically, for dipole antennas, the efficiency and performance and ease of matching the RFID chip, the antenna is related to how much dipole length can fit into a given area <NUM>. In <FIG>, therefore, without the bridge conductor, the central matching resonator <NUM> occupies a relatively large area, so the space available for the folded dipole length <NUM> is reduced, so less length can be used. For example, in an antenna of <NUM> x <NUM> using an unbridged strap (i.e., no bridge conductor), the central loop (i.e., inductor) may occupy a <NUM> x <NUM>, which is <NUM><NUM>; on the other hand, using a strap with twice the capacitance may require a central loop (i.e., inductor) occupying <NUM> x <NUM>, which is <NUM><NUM>. As a result, an area of <NUM><NUM> can be used for other elements of the antenna such as the dipole. In <FIG>, the size of the central matching resonator <NUM> is reduced by using a bridge conductor and hence more folded dipole length <NUM> can be fit in.

In an alternative embodiment shown in <FIG>, the size and shape of the shield element or bridge conductor <NUM> and <NUM> over the top of the strap pads <NUM> may be varied depending on the wants and/or needs of a user. For example, as shown in <FIG>, the area of the bridge conductor <NUM> is much larger than the area of the strap pads <NUM>, giving high bridging capacitance. In <FIG>, in contrast, the size of the bridge conductor <NUM> has been reduced and is much smaller than the area of the strap pads <NUM>, hence giving lower levels of bridging capacitance. Thus, the bridge conductor can be a variable structure regulating the bridging capacitance based on the wants and/or needs of a user.

Additionally, as shown in <FIG>, the RFID device <NUM> comprises a bridge conductor <NUM> that can be modified via a cutting process, such as via a laser or mechanical die cutting, to alter the bridging capacitance. Specifically, the cutting process is typically a cut line <NUM>, such as a laser cut line, but can be any suitable cutting process as is known in the art. The cutting process can be performed before or after the pair of strap pads <NUM> and RFID chip <NUM> are attached to an antenna (not shown). The cutting process can modify the shape and/or area of the bridge conductor <NUM> and, therefore, allow the bridging capacitance to be changed. Changing the bridging capacitance allows for tuning of the total RFID chip <NUM> and bridge capacitance.

This change in capacitance can be used to accommodate manufacturing tolerances or shift the operation frequency of an antenna between two bands. For example, for ultra-high frequency (UHF) tags Europe uses a frequency between <NUM> and <NUM> wherein the United States uses a frequency between <NUM> and <NUM>. Thus, by using a cutting process to modify the shape and/or area of the bridge conductor <NUM> to change the bridging capacitance, the same RFID device <NUM> can be used in two different bands merely by changing the bridging capacitance. Use of the same RFID device <NUM> having a bridge conductor with a variable bridging capacitance can advantageously reduce manufacturing and operational costs because it allows for the use of one RFID device design in multiple frequency bands.

Claim 1:
A radio-frequency identification, RFID, device (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising:
a first strap conductor (<NUM>, <NUM>) comprised of at least one pair of strap pads (<NUM>, <NUM>, <NUM>, <NUM>) and an RFID chip (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) connected between the at least one pair of strap pads (<NUM>, <NUM>, <NUM>, <NUM>);
an antenna conductor (<NUM>, <NUM>, <NUM>);
a second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>); and
a dielectric (<NUM>, <NUM>, <NUM>) positioned between the first strap conductor (<NUM>, <NUM>) and the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>),
characterized in that the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is a bridge conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) arranged to couple the first strap conductor (<NUM>, <NUM>) and the antenna conductor (<NUM>, <NUM>, <NUM>);
wherein the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the first strap conductor (<NUM>, <NUM>), and the antenna conductor (<NUM>, <NUM>, <NUM>) are arranged to overlap each other to provide a mutual area (<NUM>) with separating dielectrics (<NUM>, <NUM>, <NUM>) such that the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the first strap conductor (<NUM>, <NUM>), and the antenna conductor (<NUM>, <NUM>, <NUM>) are coupled via a plurality of capacitors (<NUM>, <NUM>, <NUM>) each having a capacitance value, and
wherein a capacitor (<NUM>) formed by the first strap conductor (<NUM>, <NUM>) and the antenna conductor (<NUM>, <NUM>, <NUM>) has larger capacitive value in comparison to capacitors (<NUM>, <NUM>) formed by:
the coupling between the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the first strap conductor (<NUM>, <NUM>), and
the coupling between the second conductor (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the antenna conductor (<NUM>, <NUM>, <NUM>).