Design of Paper-Substrate Dipole Antennas ... - IEEE Xplore
2011 IEEE International Conference on RFID-Technologies and Applications
Design of Paper-Substrate Dipole Antennas Magnetically Coupled to UHF RFID Silicon Chips F. Alimenti, G. Orecchini, M. Virili, V. Palazzari, P. Mezzanotte, L. Roselli University of Perugia, Dept. of Electronic and Information Engineering, via G. Duranti 93, 06125 Perugia, ITALY Abstract—This work investigates the design of a papersubstrate dipole antenna and its magnetic coupling to a UHF RFID chip. The magnetic coupling is realized by means of a heterogeneous transformer, the primary winding of which is implemented on the same paper-substrate of the antenna. The secondary winding, instead, is directly fabricated on the Si RFID chip, thus does not require for galvanic contacts between chip and antenna. The transformer insertion loss can be reduced to the device Maximum Available Gain (MAG) if a proper impedance termination of the primary (ZS,opt ) and secondary (ZL,opt ) windings is adopted. For the considered heterogeneous transformer the MAG is quite low and around −0.6 dB. Following this idea, a bow-tie dipole antenna is designed to meet ZS,opt at the operating frequency of 868 MHz. The antenna size is reduced by exploiting a meander line. As a result, the designed dipole features an overall length of about 40 mm. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized for the magnetic RFID coupling concept. Index Terms—Dipole antennas, RFID, heterogeneous integration, flexible electronics, ink-jet paper printed circuits.
Fig. 1. RFID chip magnetically coupled to a dipole antenna. Particular of the coupling transformer (top): the primary winding is fabricated on paper whereas the secondary winding is on-chip.
I. I NTRODUCTION RFID tags working in the UHF frequency range rely on low power CMOS circuits and flexible substrate antennas. In recent papers an ultra-low cost assembly process for chip and antennas has been demonstrated, promising a significant break-trough in RFID technology. To this purpose the CMOS chip is magnetically coupled to the antenna realized on a paper substrate, thus eliminating all the galvanic contacts between the chip and the antenna itself [1], [2]. The magnetic coupling is established, as in Fig. 1, by a heterogeneous planar transformer, the primary and secondary windings of which are implemented on paper substrate and Si chip respectively. As a result the RFID chip can be mounted by mere placing and gluing process steps. In particular the chip will be pad-less and completely passivated, the pad-ring being substituted by the secondary coil of the transformer. Considering a typical 1 mm2 RFID chip area, and assuming a secondary winding similar to the on-chip coil reported in [3], several heterogeneous transformer geometries have been developed. The insertion loss of these devices is quite low in the UHF frequency range (less than 1 dB) and can be minimized in order to achieve the Maximum Available Gain (MAG), that holds when optimum terminations are provided at both primary (i.e. the antenna side) and secondary (i.e. the chip side) windings.
978-1-4577-0027-9/11/$26.00 ©2011 IEEE
This paper deals with the design of a bow-tie dipole antenna on paper substrate. A meander-line matching network has been embedded within the antenna. This way, without the need for additional components, the antenna impedance can meet the optimum value required by the heterogeneous transformer. As the final step, the primary winding of the heterogeneous transformer has been integrated with the bow-tie antenna and the overall structure is validated by means of electromagnetic simulations. The obtained layout can be simply produced by exploiting the inkjet printing process with conductive silver ink. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized according to the magnetic RFID coupling concept. II. T RANSFORMER STRUCTURES The heterogeneous transformer is constituted by a primary winding on the paper substrate and by a secondary winding on the silicon chip. On the paper substrate the coil is assumed to be printed by means of an inkjet technology. The silver ink conductivity is around 2.5 × 107 S/m as in [4]. The ink thickness, instead, is around 2 μm as pointed-out in [5]. The minimum metal track width and spacing is 50 μm, corresponding to the maximum spatial resolution of the printer.
219
On silicon, a square RFID chip is considered, the side of which is 1 mm in length. The secondary windings is substantially that reported in [3]. The complete design parameters set is listed in Table I. Primary and secondary windings have been placed face-toface as in Fig. 1 and separated by a distance of 50 μm. The spacing layer has been assumed filled of air (εr = 1). Two TABLE I H ETEROGENEOUS PAPER -S I T RANSFORMERS Parameter material number of turns transformer side track width track spacing track thickness track σ substrate height
substrate εr
substrate tan δ substrate ρ
Primary paper 1–3 1.0 mm 50 μm 50 μm 2 μm 25 MS/m 260 μm
3.3
0.08 N.A.
Secondary Si chip 3 795 10 5 3 58 750 (Si bulk) 3.7 (Epi layer) 11 (SiO2 ) 11.9 (Si bulk) 11.9 (Epi layer) 4.1 (SiO2 ) N.A. 50 (Si bulk) 20 (Epi layer)
(a) ZS,opt
μm μm μm μm MS/m μm μm μm
Ω cm Ω cm
heterogeneous transformer structures have been considered. In particular, both single-turn and 3-turns primary coil have been studied. The secondary is always the same. These structures have been numerically analyzed by means of a full-wave electromagnetic simulator. The obtained results are reported in Fig. 2 showing the constant available power gain Ga and operating power gain Gp circles at the frequency of 868 MHz. From a design point of view, Fig. 2 indicates the optimum solution to the maximum power transfer problem. This means that, in order to let the transformer work at the best conditions as possible, the input impedances of both antenna and RFID circuit must be designed to meet the optimum values ZS,opt and ZL,opt , respectively. This way, the power transfer ratio from the antenna to the RFID circuit will achieve the Maximum Available Gain (MAG) of the device. At 868 MHz the simulated transformer MAG is equal to about −0.6 dB, for the single-turn primary and to about −0.5 dB, if the 3-turns primary is adopted. So the MAG is almost independent from the primary winding. The optimum source impedances, instead, are strongly dependent on the number of primary turns. The 3-turns primary, for example, shows a ZS,opt relatively close to the Smith chart center. This means that a dipole antenna can easily be designed to meet such an impedance. The disadvantage of the 3-turns primary, however, is that the layout requires two via-holes and an underpass to be fabricated [6]. On the other hand, the single-turn primary does not require via-holes and thus is an ultra-low cost solution. The main
(b) ZL,opt Fig. 2. Constant Ga (a) and Gp (b) circles, with 0.25 dB gain steps, for the two simulated paper to Si transformers: 3-turns (solid line) and singleturn (dashed line) primary. The MAG at 868 MHz is about −0.5 dB for the transformer featuring a 3-turns primary and about −0.6 dB for the single-turn primary.
drawback of the single-turn primary is a strong reduction of the optimum source impedance. At 868 MHz we have ZS,opt = 6.4 − j10.8 Ω. A bow-tie dipole antenna optimized for this case will be discussed in the next section. III. A NTENNA DESIGN METHOD Since the transformer insertion loss would be minimized if optimum terminations are provided, the main goal in the antenna design is to match the antenna input impedance to ZS,opt . This method will be illustrated in the following for the transformer featuring a single-turn primary winding. Extension to 3-turns primary coil is straightforward. It has to be noticed that, in RFID tags, it is not common to add an external matching network with lumped elements due to cost, fabrication and size issues. Therefore the matching mechanisms have to be embedded within the tags antenna layout. The proposed antenna shape, see Fig. 3, has been conceived in order to meet ZS,opt without the need of an external network. In particular, the antenna reactance was controlled
220
2l b a Fig. 3. Layout of the designed bow-tie dipole antenna on paper substrate. Main geometrical parameters: 2 l = 40, mm, a = 27 mm, b = 11.5 mm. The meander lines are used for impedance matching purposes.
by changing the length of the central meander line, while the resistance is adjusted controlling the width. The very common shape of bow-tie dipole antenna has been modified in order to save the amount of silver ink used during printing. A particular portion of solid surfaces is removed, according to [7]. Such an optimization is based on the study of the currents flowing on the antenna metal surface. The conductive ink has been removed where the current density is lower. The simulated input antenna impedance is shown in Fig. 4 for the frequency range between 750 MHz and 1000 MHz. At the design frequency of 868 MHz this impedance is very close to ZS,opt . The obtained radiation efficiency is about 51%, because the ohmic losses of the 2 μm-thick silver ink metal layer. The maximum gain and directivity are 0.8 dBi and 1.5 dBi respectively. Following the same guidelines, the bow-tie antenna was redesigned also in the case of the transformer with 3-turns primary winding. The obtained radiation efficiency is about 92%, because of a higher value of ZS,opt with respect to the singleturn primary (i.e. a shorter meander line is needed). According to such an efficiency improvement also the maximum gain improves to 1.4 dBi, this for the same calculated directivity (1.5 dBi). IV. R ESULTS In order to verify the previous design, a comprehensive electromagnetic simulation of the antenna-transformer structure is carried-out. The overall geometry has been obtained by connecting the two, previously developed, electromagnetic models of antenna and transformer. In order to reduce the border effects, tapered transitions have been conceived as shown in inset of Fig. 1. The overall structure has been validated by a FEM simulation performed under the CST Microwave Studio environment. As background material a vacuum box has been created around the overall circuit. In order to calculate the far-field radiation pattern, the box faces have been terminated on open boundary
Fig. 4. Input impedance of the designed antenna in the frequency range from 750 MHz to 1000 MHz. The constant Ga circles of the transformer with a single-turn primary winding are also shown in the figure. At the center frequency of 868 MHz the optimum source impedance ZS,opt = 6.4 − j10.8 Ω is meet by the proposed design.
conditions. The structure has been excited through a lumped port connected to the secondary winding terminals. In order to achieve the maximum power transfer, the impedance seen from these terminals (Zout ) should be equal ∗ to ZL, opt . In Fig. 5 the behavior of Zout is drawn in the frequency range from 750 MHz and 1000 MHz. The case of single-turn primary coil is considered. At the design frequency (868 MHz) Zout is close to the expected value. The difference can be ascribed to two factors: first to the presence of the tapered transitions which have been neglected in the transformer simulation and second, to the minor mismatching between the antenna impedance and ZS,opt . Nonetheless, it is evident that Zout is inside the 0.25 dB gain circle for a wide range of frequencies. Assuming a matched RFID chip, i.e. Zchip = ZL,opt , the overall insertion losses can be related to the antennatransformer radiation efficiency. Such a quantity includes the losses due to both the antenna and the transformer and can be directly evaluated using CST. The results of this study show an overall radiation efficiency of about 38% at 868 MHz. Re-designing the antenna for the transformer with 3-turns primary winding, the results of Fig. 6 are obtained. In this case the overall radiation efficiency rises to about 80%. V. C ONCLUSIONS This paper shows, for the first time, a paper-substrate dipole antenna the design of which has been optimized for magnetically coupled (i.e. without galvanic contacts) RFID chips. The coupling is achieved by means of a heterogeneous transformer having, at the operating frequency of 868 MHz, a MAG of about −0.6 dB. A single-turn primary winding has been adopted on the paper substrate in such a way that via connections and underpass are avoided. As a result the dipole antenna and the primary transformer winding can be fabricated on a ultra-low cost paper substrate using an inkjet printing process. The 3-turns secondary winding, instead, is
221
[2]
[3]
[4]
[5] [6] Fig. 5. Electromagnetic simulation of the overall structure, i.e. dipole antenna and heterogeneous paper-Si transformer, in the case of single-turn primary winding. The output impedance Zout seen from the secondary winding terminals is drawn in the frequency range from 750 MHz to 1000 MHz along with both ZL,opt = 56.7−j105.7 Ω and its complex conjugate. At the center ∗ frequency Zout is close to ZL, opt , showing the goodness of the proposed design.
[7]
Fig. 6. Zout of the overall structure versus frequency in the case of 3-turns primary winding.
directly fabricated on the Si-chip and replaces the IC pad-ring. A square coil is exploited having a 1 mm side. The design has been verified by a rigorous electromagnetic simulation of the overall antenna-transformer structure, showing a 38% radiation efficiency. Such a radiation efficiency can be improved up to about 80% adopting a 3-turns primary coil and re-designing the bow-tie antenna for the new transformer. ACKNOWLEDGMENT This work was partially supported by the COST Action IC0803 “RF/Microwave Communication Subsystems for Emerging Wireless Technologies (RFCSET)”. The authors acknowledge Agilent Technologies and Computer Simulation Technologies for the donation of the software licenses. R EFERENCES [1] A. Finocchiaro, G. Ferla, G. Girlando, F. Carrara, and G. Palmisano, “A 900-MHz RFID system with TAG-antenna magnetically-coupled to
222
the die,” in IEEE Radio Frequency Integrated Circuit Symposium, Atlanta (GA), Apr. 2008, pp. 281–284. F. Alimenti, M. Virili, G. Orecchini, P. Mezzanotte, V. Palazzari, M. M. Tentzeris, and L. Roselli, “A new contactless assembly method for paper substrate antennas and UHF RFID chips,” IEEE Trans. on Microwave Theory and Techniques, vol. 59, no. 3, pp. 627–637, Mar. 2011. X. Jingtian, Y. Na, C. Wenyi, X. Conghui, W. Xiao, Y. Yuqing, J. Hongyan, and M. Hao, “Low-cost low-power UHF RFID tag with onchip antenna,” Journal of Semiconductors, vol. 30, no. 7, pp. 075 012/1– 075 012/6, July 2009. L. Yang, A. Rida, R. Vyas, and M. M. Tentzeris, “RFID tag and RF structures on a paper substrate using inkjet-printing technology,” IEEE Transaction on Microwave Theory and Techniques, vol. 55, no. 12, pp. 2894–2901, Dec. 2007. V. Sanchez-Romaguera, M. Madec, and S. Yeates, “Inkjet printing of 3D metalinsulatormetal crossovers,” Reactive & Functional Polymers, vol. 68, pp. 1052–1058, 2008. A. C. Siegel, S. T. Phillips, M. D. Dickey, N. Lu, Z. Suo, and G. M. Whitesides, “Foldable printed circuit boards on paper substrates,” Advanced Functional Materials, vol. 20, no. 1, pp. 28–35, Jan. 2010. G. Orecchini, L. Yang, A. Rida, F. Alimenti, M. M. Tentzeris, and L. Roselli, “Green technologies and RFID: Present and future,” Applied Comput. Electromagnetics Society Journal, vol. 25, no. 3, pp. 230–238, Mar. 2010.
Design of Paper-Substrate Dipole Antennas Magnetically Coupled to UHF RFID Silicon Chips F. Alimenti, G. Orecchini, M. Virili, V. Palazzari, P. Mezzanotte, L. Roselli University of Perugia, Dept. of Electronic and Information Engineering, via G. Duranti 93, 06125 Perugia, ITALY Abstract—This work investigates the design of a papersubstrate dipole antenna and its magnetic coupling to a UHF RFID chip. The magnetic coupling is realized by means of a heterogeneous transformer, the primary winding of which is implemented on the same paper-substrate of the antenna. The secondary winding, instead, is directly fabricated on the Si RFID chip, thus does not require for galvanic contacts between chip and antenna. The transformer insertion loss can be reduced to the device Maximum Available Gain (MAG) if a proper impedance termination of the primary (ZS,opt ) and secondary (ZL,opt ) windings is adopted. For the considered heterogeneous transformer the MAG is quite low and around −0.6 dB. Following this idea, a bow-tie dipole antenna is designed to meet ZS,opt at the operating frequency of 868 MHz. The antenna size is reduced by exploiting a meander line. As a result, the designed dipole features an overall length of about 40 mm. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized for the magnetic RFID coupling concept. Index Terms—Dipole antennas, RFID, heterogeneous integration, flexible electronics, ink-jet paper printed circuits.
Fig. 1. RFID chip magnetically coupled to a dipole antenna. Particular of the coupling transformer (top): the primary winding is fabricated on paper whereas the secondary winding is on-chip.
I. I NTRODUCTION RFID tags working in the UHF frequency range rely on low power CMOS circuits and flexible substrate antennas. In recent papers an ultra-low cost assembly process for chip and antennas has been demonstrated, promising a significant break-trough in RFID technology. To this purpose the CMOS chip is magnetically coupled to the antenna realized on a paper substrate, thus eliminating all the galvanic contacts between the chip and the antenna itself [1], [2]. The magnetic coupling is established, as in Fig. 1, by a heterogeneous planar transformer, the primary and secondary windings of which are implemented on paper substrate and Si chip respectively. As a result the RFID chip can be mounted by mere placing and gluing process steps. In particular the chip will be pad-less and completely passivated, the pad-ring being substituted by the secondary coil of the transformer. Considering a typical 1 mm2 RFID chip area, and assuming a secondary winding similar to the on-chip coil reported in [3], several heterogeneous transformer geometries have been developed. The insertion loss of these devices is quite low in the UHF frequency range (less than 1 dB) and can be minimized in order to achieve the Maximum Available Gain (MAG), that holds when optimum terminations are provided at both primary (i.e. the antenna side) and secondary (i.e. the chip side) windings.
978-1-4577-0027-9/11/$26.00 ©2011 IEEE
This paper deals with the design of a bow-tie dipole antenna on paper substrate. A meander-line matching network has been embedded within the antenna. This way, without the need for additional components, the antenna impedance can meet the optimum value required by the heterogeneous transformer. As the final step, the primary winding of the heterogeneous transformer has been integrated with the bow-tie antenna and the overall structure is validated by means of electromagnetic simulations. The obtained layout can be simply produced by exploiting the inkjet printing process with conductive silver ink. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized according to the magnetic RFID coupling concept. II. T RANSFORMER STRUCTURES The heterogeneous transformer is constituted by a primary winding on the paper substrate and by a secondary winding on the silicon chip. On the paper substrate the coil is assumed to be printed by means of an inkjet technology. The silver ink conductivity is around 2.5 × 107 S/m as in [4]. The ink thickness, instead, is around 2 μm as pointed-out in [5]. The minimum metal track width and spacing is 50 μm, corresponding to the maximum spatial resolution of the printer.
219
On silicon, a square RFID chip is considered, the side of which is 1 mm in length. The secondary windings is substantially that reported in [3]. The complete design parameters set is listed in Table I. Primary and secondary windings have been placed face-toface as in Fig. 1 and separated by a distance of 50 μm. The spacing layer has been assumed filled of air (εr = 1). Two TABLE I H ETEROGENEOUS PAPER -S I T RANSFORMERS Parameter material number of turns transformer side track width track spacing track thickness track σ substrate height
substrate εr
substrate tan δ substrate ρ
Primary paper 1–3 1.0 mm 50 μm 50 μm 2 μm 25 MS/m 260 μm
3.3
0.08 N.A.
Secondary Si chip 3 795 10 5 3 58 750 (Si bulk) 3.7 (Epi layer) 11 (SiO2 ) 11.9 (Si bulk) 11.9 (Epi layer) 4.1 (SiO2 ) N.A. 50 (Si bulk) 20 (Epi layer)
(a) ZS,opt
μm μm μm μm MS/m μm μm μm
Ω cm Ω cm
heterogeneous transformer structures have been considered. In particular, both single-turn and 3-turns primary coil have been studied. The secondary is always the same. These structures have been numerically analyzed by means of a full-wave electromagnetic simulator. The obtained results are reported in Fig. 2 showing the constant available power gain Ga and operating power gain Gp circles at the frequency of 868 MHz. From a design point of view, Fig. 2 indicates the optimum solution to the maximum power transfer problem. This means that, in order to let the transformer work at the best conditions as possible, the input impedances of both antenna and RFID circuit must be designed to meet the optimum values ZS,opt and ZL,opt , respectively. This way, the power transfer ratio from the antenna to the RFID circuit will achieve the Maximum Available Gain (MAG) of the device. At 868 MHz the simulated transformer MAG is equal to about −0.6 dB, for the single-turn primary and to about −0.5 dB, if the 3-turns primary is adopted. So the MAG is almost independent from the primary winding. The optimum source impedances, instead, are strongly dependent on the number of primary turns. The 3-turns primary, for example, shows a ZS,opt relatively close to the Smith chart center. This means that a dipole antenna can easily be designed to meet such an impedance. The disadvantage of the 3-turns primary, however, is that the layout requires two via-holes and an underpass to be fabricated [6]. On the other hand, the single-turn primary does not require via-holes and thus is an ultra-low cost solution. The main
(b) ZL,opt Fig. 2. Constant Ga (a) and Gp (b) circles, with 0.25 dB gain steps, for the two simulated paper to Si transformers: 3-turns (solid line) and singleturn (dashed line) primary. The MAG at 868 MHz is about −0.5 dB for the transformer featuring a 3-turns primary and about −0.6 dB for the single-turn primary.
drawback of the single-turn primary is a strong reduction of the optimum source impedance. At 868 MHz we have ZS,opt = 6.4 − j10.8 Ω. A bow-tie dipole antenna optimized for this case will be discussed in the next section. III. A NTENNA DESIGN METHOD Since the transformer insertion loss would be minimized if optimum terminations are provided, the main goal in the antenna design is to match the antenna input impedance to ZS,opt . This method will be illustrated in the following for the transformer featuring a single-turn primary winding. Extension to 3-turns primary coil is straightforward. It has to be noticed that, in RFID tags, it is not common to add an external matching network with lumped elements due to cost, fabrication and size issues. Therefore the matching mechanisms have to be embedded within the tags antenna layout. The proposed antenna shape, see Fig. 3, has been conceived in order to meet ZS,opt without the need of an external network. In particular, the antenna reactance was controlled
220
2l b a Fig. 3. Layout of the designed bow-tie dipole antenna on paper substrate. Main geometrical parameters: 2 l = 40, mm, a = 27 mm, b = 11.5 mm. The meander lines are used for impedance matching purposes.
by changing the length of the central meander line, while the resistance is adjusted controlling the width. The very common shape of bow-tie dipole antenna has been modified in order to save the amount of silver ink used during printing. A particular portion of solid surfaces is removed, according to [7]. Such an optimization is based on the study of the currents flowing on the antenna metal surface. The conductive ink has been removed where the current density is lower. The simulated input antenna impedance is shown in Fig. 4 for the frequency range between 750 MHz and 1000 MHz. At the design frequency of 868 MHz this impedance is very close to ZS,opt . The obtained radiation efficiency is about 51%, because the ohmic losses of the 2 μm-thick silver ink metal layer. The maximum gain and directivity are 0.8 dBi and 1.5 dBi respectively. Following the same guidelines, the bow-tie antenna was redesigned also in the case of the transformer with 3-turns primary winding. The obtained radiation efficiency is about 92%, because of a higher value of ZS,opt with respect to the singleturn primary (i.e. a shorter meander line is needed). According to such an efficiency improvement also the maximum gain improves to 1.4 dBi, this for the same calculated directivity (1.5 dBi). IV. R ESULTS In order to verify the previous design, a comprehensive electromagnetic simulation of the antenna-transformer structure is carried-out. The overall geometry has been obtained by connecting the two, previously developed, electromagnetic models of antenna and transformer. In order to reduce the border effects, tapered transitions have been conceived as shown in inset of Fig. 1. The overall structure has been validated by a FEM simulation performed under the CST Microwave Studio environment. As background material a vacuum box has been created around the overall circuit. In order to calculate the far-field radiation pattern, the box faces have been terminated on open boundary
Fig. 4. Input impedance of the designed antenna in the frequency range from 750 MHz to 1000 MHz. The constant Ga circles of the transformer with a single-turn primary winding are also shown in the figure. At the center frequency of 868 MHz the optimum source impedance ZS,opt = 6.4 − j10.8 Ω is meet by the proposed design.
conditions. The structure has been excited through a lumped port connected to the secondary winding terminals. In order to achieve the maximum power transfer, the impedance seen from these terminals (Zout ) should be equal ∗ to ZL, opt . In Fig. 5 the behavior of Zout is drawn in the frequency range from 750 MHz and 1000 MHz. The case of single-turn primary coil is considered. At the design frequency (868 MHz) Zout is close to the expected value. The difference can be ascribed to two factors: first to the presence of the tapered transitions which have been neglected in the transformer simulation and second, to the minor mismatching between the antenna impedance and ZS,opt . Nonetheless, it is evident that Zout is inside the 0.25 dB gain circle for a wide range of frequencies. Assuming a matched RFID chip, i.e. Zchip = ZL,opt , the overall insertion losses can be related to the antennatransformer radiation efficiency. Such a quantity includes the losses due to both the antenna and the transformer and can be directly evaluated using CST. The results of this study show an overall radiation efficiency of about 38% at 868 MHz. Re-designing the antenna for the transformer with 3-turns primary winding, the results of Fig. 6 are obtained. In this case the overall radiation efficiency rises to about 80%. V. C ONCLUSIONS This paper shows, for the first time, a paper-substrate dipole antenna the design of which has been optimized for magnetically coupled (i.e. without galvanic contacts) RFID chips. The coupling is achieved by means of a heterogeneous transformer having, at the operating frequency of 868 MHz, a MAG of about −0.6 dB. A single-turn primary winding has been adopted on the paper substrate in such a way that via connections and underpass are avoided. As a result the dipole antenna and the primary transformer winding can be fabricated on a ultra-low cost paper substrate using an inkjet printing process. The 3-turns secondary winding, instead, is
221
[2]
[3]
[4]
[5] [6] Fig. 5. Electromagnetic simulation of the overall structure, i.e. dipole antenna and heterogeneous paper-Si transformer, in the case of single-turn primary winding. The output impedance Zout seen from the secondary winding terminals is drawn in the frequency range from 750 MHz to 1000 MHz along with both ZL,opt = 56.7−j105.7 Ω and its complex conjugate. At the center ∗ frequency Zout is close to ZL, opt , showing the goodness of the proposed design.
[7]
Fig. 6. Zout of the overall structure versus frequency in the case of 3-turns primary winding.
directly fabricated on the Si-chip and replaces the IC pad-ring. A square coil is exploited having a 1 mm side. The design has been verified by a rigorous electromagnetic simulation of the overall antenna-transformer structure, showing a 38% radiation efficiency. Such a radiation efficiency can be improved up to about 80% adopting a 3-turns primary coil and re-designing the bow-tie antenna for the new transformer. ACKNOWLEDGMENT This work was partially supported by the COST Action IC0803 “RF/Microwave Communication Subsystems for Emerging Wireless Technologies (RFCSET)”. The authors acknowledge Agilent Technologies and Computer Simulation Technologies for the donation of the software licenses. R EFERENCES [1] A. Finocchiaro, G. Ferla, G. Girlando, F. Carrara, and G. Palmisano, “A 900-MHz RFID system with TAG-antenna magnetically-coupled to
222
the die,” in IEEE Radio Frequency Integrated Circuit Symposium, Atlanta (GA), Apr. 2008, pp. 281–284. F. Alimenti, M. Virili, G. Orecchini, P. Mezzanotte, V. Palazzari, M. M. Tentzeris, and L. Roselli, “A new contactless assembly method for paper substrate antennas and UHF RFID chips,” IEEE Trans. on Microwave Theory and Techniques, vol. 59, no. 3, pp. 627–637, Mar. 2011. X. Jingtian, Y. Na, C. Wenyi, X. Conghui, W. Xiao, Y. Yuqing, J. Hongyan, and M. Hao, “Low-cost low-power UHF RFID tag with onchip antenna,” Journal of Semiconductors, vol. 30, no. 7, pp. 075 012/1– 075 012/6, July 2009. L. Yang, A. Rida, R. Vyas, and M. M. Tentzeris, “RFID tag and RF structures on a paper substrate using inkjet-printing technology,” IEEE Transaction on Microwave Theory and Techniques, vol. 55, no. 12, pp. 2894–2901, Dec. 2007. V. Sanchez-Romaguera, M. Madec, and S. Yeates, “Inkjet printing of 3D metalinsulatormetal crossovers,” Reactive & Functional Polymers, vol. 68, pp. 1052–1058, 2008. A. C. Siegel, S. T. Phillips, M. D. Dickey, N. Lu, Z. Suo, and G. M. Whitesides, “Foldable printed circuit boards on paper substrates,” Advanced Functional Materials, vol. 20, no. 1, pp. 28–35, Jan. 2010. G. Orecchini, L. Yang, A. Rida, F. Alimenti, M. M. Tentzeris, and L. Roselli, “Green technologies and RFID: Present and future,” Applied Comput. Electromagnetics Society Journal, vol. 25, no. 3, pp. 230–238, Mar. 2010.
