Capacitive antenna and method for making same

Translation of description of equivalent WO 2004012299 A2

capacitive antenna and method of making

The present invention relates to a capacitive antenna and a method for making such an antenna. She especially finds its use in the field of applications related to wireless communication technologies, including radio frequency identification applications (RFID). These applications are for example implemented for automatic identification and data transmission in the areas of access control and management of electronic data. In the area of ​​access control and or electronic purse, the applications are for example in the context of public transport tickets, motorway tolls, parking tickets, air tickets. Many companies have also developed identification means for their personnel or their customer card contactless smart.

two main frequency bands harnessed to date there are to radio frequency identification applications: low frequency around 125 kHz and medium frequencies around 13.56 MHz. The values ​​of these frequencies are generally fixed and correspond to international standards. To implement this technology, mainly used a playback device capable of communicating with a portable device carried by a user. Communication takes place via remote electromagnetic coupling between an antenna housed in the mobile device and a second antenna located in the reading device.

The mobile device, or transponder, generally comprises a support on which are provided an electronic device for developing, storing and processing information, for example a chip, and the first antenna with which the device is connected. It is generally in the form of a credit card in ISO format or a flexible label ( "Tag").

Overall, the price of a chip is proportional to the surface area of ​​silicon used to house the microprocessor, memory areas and capacitors. To reduce significantly the cost of the antenna and the chip micropackaging it is known in the art of seek to reduce the chip size by reducing the clutter generated by the capacitors. therefore uses chips containing compact capacitors with lower capacity.

Therefore, in parallel to the decrease in chip size, inductance of the antenna constant, it becomes necessary that the support also another capacitor so that the device of the resonance law is observed. The optimal operation of the device is obtained at resonance, where the characteristics of the different components of this device respect the following law of resonance: l_aC_pω² = 1 where

L_at corresponds to the inductance of the antenna, C_p refers to the ability of the device, and

03 = 2.pi.f corresponds to the pulse and is calculated depending on the frequency (f) selected for the exchange of signals. As described in WO-A-01/50547, it is known to provide a second capacitor in parallel with the chip and the antenna. This second capacity compensates for the fact that the chip capacity is less. In particular this document teaches screen printing of the capacitor in the same way that the antenna is screen printed. Screen printing is derived from the stencil printing technique. This is a printing process using a screen constituted by a frame on which is stretched a knitted fabric. The fabric is usually made of synthetic fiber such as nylon or polyester. This screen, applied on the support, which receives the ink, pushed by the scraper passes through the free meshes to perform printing. The thickness of the printed deposit is irregular.

The devices of the prior art a problem. Indeed, they allow the use of smaller chips and thus less expensive, but against these devices impose certain constraints on the achievement of the antenna. The antenna is screen printed on a support. Generally the antenna comprises a plurality of turns such that the first pad of the antenna is located inside the turns, while the second antenna pad is located outside the turns. For connecting the chip and the second capacitor in parallel with the antenna, it is necessary to connect the capacitor to each of the two pads of the antenna. The problem is essentially posed in the prior art that the antenna must include several turns, given the capacity of the capacitors and the resonance law to respect. The second capacitor is screen printed on the outside of the center of the turns to avoid harming flow through and thus the inductance of the antenna. Therefore this second capacitor is easily connected to the external pad of the antenna. To connect to the inside pad of the antenna, it is necessary to provide an insulating bridge over the turns at which a conductive connection can then be screen printed. The completion of this bridge is binding and adds additional steps to the antenna manufacturing process. With the screen printing technique, capacitors that can be obtained have an intermediate capacity. This ability does not come fully complete the reduction of the internal capacity of the chip. Therefore that the resonance law is observed, it is necessary to increase the inductance of the antenna, which is obtained by increasing the number of turns, and by imposing the achievement of a bridge to connect this antenna multi-turn on the second screen printed capacitor.

In the prior art discloses capacitors having a higher capacity, and which may cooperate with a single turn antenna. But in this case such capacitors are expensive, bulky and negate cost reduction efforts.

The invention aims to solve the problems mentioned and enables the production of planar antennas with low cost and high volume taking into account the future technical constraints imposed by the chip manufacturers. According to the invention it is possible to provide on the same support an antenna preferably having a single turn, this antenna being connected to a capacitor of high capacitance. The capacity of a plane capacitor is deduced from the following equation: C = ε₀^* B_r * S / E where

C is the value of the capacitance, ε₀ is the dielectric permittivity of vacuum (8.854. 10¹² F / m), 8_r corresponds to the relative permittivity of the dielectric, S corresponds to the surface electrodes vis-a-vis one another, and e is the dielectric thickness. In the invention, there is obtained a high capacity capacitor playing mainly on the value of the dielectric thickness that is disposed between the two conductive plates. To obtain the result of the invention, the capacitor is printed by gravure printing on the support also having the antenna. Indeed, by the technique of photogravure, the deposition layer of very small thickness is obtained. The capacitor is obtained by depositing at least three superposed and successive layers such as a first conductive layer, covered with a second insulating layer, and finally itself covered by a third conductive layer. For example, the antenna can -she even be printed by gravure on this occasion, the antenna design is finalized with the two conductive layers.

The gravure printing is a technique derived from the intaglio. The printing inks are hollow. The image areas are engraved on a steel cylinder coated with copper and chrome. We can use chemical solutions for etching copper. There are also machines that mechanically engrave cylinders using a diamond tip from an electronic scan of a photograph to be reproduced. Finally, another method of preparing the printing cylinders uses a laser for engraving. When printing, the ink is filled in the cylinder of the cells; a doctor blade removes excess ink and the support is then pressed against the printing form to complete the draw. The resulting impression is of a high quality and is perfectly reproducible. The gravure uses fluid inks containing volatile solvents. Even for deposits thin, you get a deposit covering evenly throughout the surface to be printed.

The benefits of this process can guarantee a constant geometry of planar capacitor. Since this capacitor has a high capacity, one single turn antenna is tuned to resonance. Therefore the capacitor and the chip can be very easily connected to the single turn antenna. The overall electrical resistance of the single turn antenna is less than the resistance of a conventional spiral. This allows consideration alternatively, an electrolytic copper deposition at high speed with a constant thickness and controlled, over each of the areas having a portion of conductive layer. Thus, the inventive method reduces significantly the cost of transponder playing both on the direct manufacturing cost of the antenna and on the simplification of micropackaging of the chip.

The invention relates to a coupling antenna comprising at least one turn presented on a support and connected to a capacitor made on the same medium, the capacitor being mounted in parallel on two pads of the antenna, characterized in that antenna and capacitor are printed by gravure printing on the same media.

The invention also relates to an embodiment of an antenna method comprising at least one coil connected to a capacitor, the antenna and the capacitor being presented on the same insulating support, characterized in that it comprises the following steps:

- Carrying out a first photogravure printing with a conductive ink to achieve an open turn of the antenna, a lower electrode of the capacitor, and a connection between a first pad of the antenna and the lower electrode,

- Performing a second gravure printing ink with a dielectric for covering the lower electrode by an insulating layer,

- Make a third gravure printing with a conductive ink for superior capacitor electrode covering the insulating layer and to obtain a connection between a second pad of the antenna and the upper electrode.

The invention will be better understood from reading the following description and examining the accompanying figures. These figures are given for guidance only and in no way limit the invention. The figures show:

Figure 1a: a top view of a medium after a first process step according to the invention,

Figure 1 b: a top view of a substrate after a second process step according to the invention, Figure 1 c is a view from above of a support after a third process step according to the invention,

Figure 1d is a view from above of a support after a final optional step of the method according to the invention,

Figure 2 is an overall view of an antenna according to the invention cooperating with a reading device. Figure 2 shows a portable device 1 intended to exchange radio signals with a reading device 2. The mobile device 1 is a transponder comprising an electronic microcircuit 3 or 3 chip and an antenna 4. For example, the chip 3 and the antenna 4 are presented on an insulating substrate 5. the substrate 5 can for example have the form of a standardized smart card in ISO format. The chip 3 is connected to the antenna 4, and is fed by the induced current generated by the electro-magnetic field transmitted and received in the antenna 4.

The reading device 2 comprises a second antenna 6 for transmitting and receiving direction of the mobile device signals 1. Furthermore, the device 2 comprises a coupler 7 connected to the second antenna 6, the coupler 7 being also connected to a unit 8 processing and management of data exchanged. The unit 8 is for example a computer.

The antenna 4 comprises according to the invention, as shown in Figures 1a, 1b, 1c and 1d, at least one coil 9 and a capacitor 10 connected in parallel to the coil 9. The coil 9 and the capacitor are shown in 0 a support 11. the support 11 is insulating and may for example be in the form of a flexible thin film. For example, the substrate 11 is polyethylene type (PE), polyester (PET), polyvinyl chloride (PVC), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), glass epoxy, polyimide, paper, etc.

The coil 9 has a first pad 12 and second pad 13 to which the capacitor 10 and the chip 3 are connected.

During a first step of the method embodiments according to the invention of the antenna 4, it has the support 11 at a first gravure cylinder supplied with electrically conductive ink. Thus, a first pattern drawing the turn 9, a lower electrode 14 of the capacitor 10, and a link 15 between the first pad 12 and the lower electrode 14. The second block 13 is already apparent from the deposition of the first layer conductive ink. For example, the thickness of the ink deposit, once dried, is of the order of 2 to 4 micrometer.

To form the capacitor 10, a second layer 16 is deposited with a dielectric material above the lower electrode 14. According to the invention, this second layer 16 is deposited by gravure printing by means of a second cylinder supplied with ink properties insulating. Preferably this second layer is obtained after a double pass under two cylinders such as the second cylinder. And the dielectric layer 16 is obtained by two superposed layers of insulating ink. With such a double thickness of the insulating layers, porosity problems are avoided in the dielectric between the lower electrode 14 of the upper electrode 17.

Typically, the thickness of the insulating layer 16 is less than 10 micrometers, and preferably ranges between 5 and 10 microns, this layer 16 being preferably obtained in two successive layers in order to limit the generatrices porosities current leakage. The dielectric layer is homogeneous and does not contain pores in which impurities could collect.

With the technology of gravure printing, and the ink used specific, the layer 16 may alternatively be obtained in a single pass under the second cylinder.

Then, to finish the capacitor 10, as presented in Figure 1c, a third layer is deposited to form the upper electrode 17, and also a connection 18 between the upper electrode 17 and the second pad 13. The third layer is printed by rotogravure using a conductive ink. In this case, preferably using a four-color machine having four cylinders in a same line.

Preferably the same conductive ink is used to produce the first layer and the third layer, the ink used in the invention has a very low electrical resistance, it contains copper, silver, gold, palladium , tin or alloys thereof as well as conductive polymers. The electrically conductive ink has to be prepared, from the viewpoint of its viscosity and from the viewpoint other physicochemical properties so that it is suitable for gravure printing. The ink chosen is for example a metal charged electrically conductive ink. In this case the metal is mainly silver, and it is presented in the form of flakes forming micro plates. These micro plates are preferably very thin (1 to 2 .mu.m) and a length of between 2 and 5 microns. The proportion of these metal charges is between 50% and 80% of the solid mass of the ink. Preferably the proportion of the metal charges is 70% to ensure a high conductivity of the ink thus formed. The ink conductivity is high in consideration of low resistivity, thereby facilitating the following metallization step. Alternatively, the ink may comprise conductive organic polymers. The advantage of these polymers is that they are formulated in a solvent or aqueous phase which allows to adjust the rheological properties of the ink obtained to make it particularly compatible with the gravure method. Another advantage is that in this embodiment, the ink does not contain metal fillers, which contributes to lower cost of large-scale, and which facilitates a homogeneous ink to the reliability of the method manufacturing.

During a last step, it is for example depositing a metal layer 19 for covering all portions of the conductive ink having either the first passage or the third passage. This metallic layer can be deposited by electrolytic copper plating. The deposited copper thickness is of the order of 5 microns and covers the coil 9, the pads 12 and 13, the links 15 and 18 and also the upper face 17 of the upper electrode of the capacitor 10. Preferably, for tune the antenna 4 with the chip 3, at a frequency of 13.56 MHz is selected a coil 9 a width of 500 microns such that it has an inductance of 270 nH. Then depending on the internal capacity of the chip 3, the capacity of the external planar capacitor is determined 10 that must be provided on the support 11. For example, in the case where the capacity of the chip 3 is 97 pF, knowing obtainable reliably a thickness of 8 micrometers for the dielectric, selecting a diameter of the electrodes equal to 11, 8 mm. Alternatively, if the capacity of the chip 3 is 25 pF, then it is necessary that the plane capacitor 10 has a capacitance of 485 pF, and for this purpose, when a dielectric thickness of 8 mm, is provides a capacitor surface such that the diameter is 12.8 mm.

Alternatively, in the invention, especially if only one dielectric layer 16 is sufficient, then the thickness being less may be provided antenna models for electronic tags with capacitors 10 very compact.