Design and Characterization ofNovel Paper-based Inkjet-Printed RFIDand Microwave Structures for Telecommunication and Sensing

Applications

Li Yang and Manos M. Tentzeris

Georgia Electronic Design Center, School of Electrical and Computer Engineering,Georgia Institute of Technology, Atlanta, GA 30332-0250, USA

E-mail: liyang@ece.gatech.edu

Abstract - In this paper, inkjet-printed microwave circuitsfabricated on paper-based substrates were investigated, as asystem-level solution for ultra-low-cost mass production. The RFcharacteristics of the paper-based substrate were studied byusing the cavity resonator method and the Transmission Linemethod in order to characterize the dielectric constant (er) andloss tangent (tan6) of the substrate. A UHF RFID tag module wasthen developed with the inkjet-printing technology which couldfunction as a technology for much simpler and faster fabricationon/in paper. Simulation and well-agreed measurement resultsverify a good performance of the tag module. In addition, for thefirst time, the possibility of paper-based substrate for multilayermicrowave structures was explored, and a 2.4GHz multilayer patchresonator bandpass filter with insertion loss < 0.6dB wasdemonstrated. These results show that the paper material can servefor the purpose of economical multilayer structures fortelecommunication and sensing applications.

Index Terms - RFID, sensor, organic-substrate, dielectriccharacterization, multilayer, inkjet-printing, paper.

I. INTRODUCTION

As the demand for low cost, flexible and efficientelectronics increases, the materials and integration techniquesbecome more and more critical and face more challenges,especially with the ever growing interest for "cognitiveintelligence" and wireless applications, such as RadioFrequency Identification (RFID) and Wireless Local AreaNetworks (WLAN), such as WiFi and WiMax. For an optimalRF performance, substrate material characterization forelectronics such as antennas, filters and transmission linesbecomes a must, especially for low-cost materials.

In this paper, for the first time, paper-based electronics areinvestigated for UHF frequency band. Paper is considered oneof the best organic-substrate candidates for UHF andmicrowave applications such as RFID/sensing. It is not onlyenvironmentally friendly, but can also undergo large reel-to-reel processing. In terms of mass production and increaseddemand, this makes paper the lowest cost material made.Paper also has low surface profile with appropriate coating.This is very crucial since fast printing processes, such asdirect write methodologies, can be utilized instead of metaletching techniques. A fast process, like inkjet printing, can beused efficiently to print electronics on/in paper substrates.

In this effort, a cavity resonator method was utilized for thedielectric constant characterization, and a Transmission Line(TL) method was utilized for the loss tangent (tan6)extraction. An RFID tag module using inductively couplingmechanism for the matching of the antenna to the IntegratedCircuit (IC) was designed and printed on a paper substrateusing conductive paste inkjet-printing, and characterizedexperimentally. In addition, a two-layer 2.4 GHz patchresonator bandpass filter (BPF) for WLAN applications wasdesigned and printed on a multilayer paper configuration.

II. RF CHARACTERISTICS OF PAPER-BASED SUBSTRATE

A. Dielectric Constant

The most precise methods for determining the substratedielectric constant are resonator methods, such as microstripring resonators, parallel plate resonators and cavity resonators[1]. The cavity resonator method doesn't require apretreatment on the substrate, and provides a higher levelaccuracy compared with the other methods.

(a)

Unloaded Cavity TE( Loaded Cavity TEOI1Af=2.2% (b)

Fig. 1. (a) Photograph of the split-cylinder cavity in unloaded andloaded status. (b) The simulated field distributions to help identifyingthe correct resonant peak corresponding to TEO1, mode.

1-4244-0688-9/07/$20.00 C 2007 IEEE 1633

-20

35

-8024 26 28 30 32 34 36 38

Frequency (GHz)

Fig. 2. Measured modes shifting of the unloaded/loaded split-cylinder cavity.

A split-cylinder resonator was fabricated with a circular-cylindrical cavity of radius 6.58mm and length 7.06mm,separated into two halves by a variable gap height which isadjustable to the thickness of the paper substrate beingcharacterized. The feeding structure is composed of coaxialcables terminated in coupling loops. A TEO1, resonant modewas excited in the cavity at

fI I

\3x108 8317 )2 + ( IT )2 (1)2 a L

where a is the cavity radius and L is its length.A hydrophobic paper was selected for the convenience of

inject-printing and to enhance durability, and placed in thegap between the two cylindrical-cavity sections. The thicknessof the paper was 263um. The existence of the insertedsubstrate caused the shifting of the TEO1, resonant mode.Using the resonance and boundary conditions for the electricand magnetic fields, the substrate's dielectric constant can becalculated from the shifting [2]. The full wave electromagneticsolver HFSS was used to assist identifying the correct positionof the TEO1, resonant peak, as shown in Fig. 1.The measurement data of the resonant modes shifting is

plotted in Fig. 2. For the empty cavity, the dominant modeTEO1, was observed at 34.54GHz. After the paper sample wasinserted, the TEO1, shifted down to 33.78GHz. In this way, thesample dielectric constant of 6r= 1.6 was determined. In thecase where paper needs to be compressed, such as inmultilayer configurations or in a laminating process, thedielectric constant will increase with increasing density. Toverify this, the same paper sample was laminated to 175umthickness, and 6r= 2.2 was observed. Since the previouscharacterization of other organic substrates performed byNational Institute of Standards and Technology (NIST)features a very flat response of relative permittivity over widefrequency ranges [3], these results are also expected to beeffective for very wide frequency bands.

B. Loss Tangent

In order to extract the total losses of the dielectric property

E 0.3m

0.1

o

0 1 2 3 4

Frequency (GHz)

Fig. 3. Measured dielectric loss of a hydrophobic paper usingTransmission Line method.

of paper, a TL method was utilized. In this method thedielectric loss of a microstrip line ad can be extracted, whichprovides the tan6 according to the formula [1]

tan6 adk e(6r - 1) (2)26r(C'e -1)

where ko is the free space wavelength, 6e is the effectivedielectric constant, and c, is the relative dielectric constant.Simulation results for conductor and radiation losses, ac and a,respectively, of the microstrip lines were subtracted from thetotal loss atot. This was done by simulating a microstrip linewith no dielectric loss in HFSS and extracting a, and ac, thensubtracting these effects from the total measured loss.Two microstrip lines of lengths 111.8mm and 74.8mm, with

width 2.53mm were measured on the same paper material of1.2mm thickness. Since the conductivity of the conductive inkis a variable of the curing temperature, 18um thick copper wasselected as the metallic material on the paper substrate at thismeasurement only, in order to accurately model and de-embedthe conductive loss of the microstrip lines.Measurements were recorded over a frequency range from

0.lGHz to 4GHz using Agilent 8530A Vector NetworkAnalyzer (VNA). The feeding to the microstrip lines wereperformed with typical SMA connectors. The calibrationmethod used was SOLT. Fig. 3 shows the plot of attenuationagainst frequency. The two measurement results agree well.The attenuation increases as expected with frequency. Theattenuation was 0.061dB/cm and 0.170dB/cm at frequencies0.9GHz and 2.4GHz respectively. These values result in atan6 = 7.7x10-2 at 0.9GHz and tan6 = 8.2x10-2 at 2.4 GHzaccording to equation (2).

III. INKJET-PRINTED MICROWAVE CIRCUITS

As a first step to an all-printed paper-based electronic circuitoperating at the UHF frequency band, two modules weredesigned and fabricated using conductive paste inkjet-printingtechnology. The two designs: RFID tag module and WLANBPF are demonstrated below.

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0.4

I LI= 77mm

19mm2| {25.2mm

L2= 18.5rMM

IC TerminalsFin

Fig. 4. Inductively coupled feeding R-FID tag module configuration.

LZ.

Rloop M Rrb* Rsub*

CrbLloop )C Lrb _ _

Fig. 5. Lumped element model for the inductively coupled feedingRFID tag module.

A. RFID Tag Module

The demand for flexible RFID tags has rapidly increased dueto the requirements of automatic identification in item-leveltracking. The major challenges existing in today's RFIDtechnology advancing toward the practical level is to lower thecost of the RFID tags and reduce the design and fabricationcycle. The characterization of paper-based substrate providesthe possibility to dramatically reduce the cost not only ofRFID's, but also of "cognitive intelligence" sensors.Most available commercial RFID tags are passive, and the

antenna translates electromagnetic waves from the reader intopower supplied to the IC. Thus, a conjugate impedancematching between the antenna and the tag IC is highlyessential to power up the IC and maximize the effective range.However, most RFID ICs primarily exhibit complex inputimpedance with large imaginary part and very small real part,making miniaturized matching extremely challenging.An inductively coupled feeding structure, as the proposed

one shown in Fig. 4, is an effective way for impedancematching. It consists of a feed loop with two terminals and aradiating body. The two terminals of the loop are connected tothe IC, and the feed "communicates" with the radiating bodythrough mutual coupling.The inductively coupled feeding structure can be modeled

as a transformer. Fig. 5 depicts the equivalent lumped elementmodel, where Rrb and Rloop are the individual resistances of theradiating body and the feed loop. M is the mutual inductanceand Lloop is the self-inductance of the feed loop. Rsu1b and Csu1bare representing the substrate effects. Without loss ofgenerality and assuming that the substrate effect is minimal,the components of antenna input resistance Rzin and reactanceXzin at the resonant frequency can be predicted from the lumpedelement modeling,

Fig. 6. Photograph of the designed RFID tag with DimatixMaterials Printer during inkjet-printing process.

400

E 300

0a)

o 200

E 100

o

0.89 0.9 0.91 0.92 0.93

Frequency (GHz)Fig. 7. Measured and simulated input resistance and reactance ofthe inkjet-printed RFID tag.

R _(2Of0M)2R.in + RloopznRrb

(3)

X1i, = 2'zT0fL,00 (4)

Thus, at the resonant frequency, the resistance is mainlycontrolled by M and Rrb, and the reactance is dependent onlyupon Lloop. In this way, the antenna input resistance and inputreactance can be adjusted independently. Therefore,inductively coupled feeding structures present one of thetheoretically optimum solutions to effectively match an

antenna to arbitrary IC impedances without the need of largematching circuits.An inductively coupled feeding antenna was inkjet-printed

with Dimatix Materials Printer DMP-2800, as shown in Fig.6. The target RFID IC was Philips EPC 1.19 Gen2 RFIDASIC IC which has a stable impedance performance of 16-j350 Q over 902 928MHz, covering the North America UHFRFID Band. The substrate was the previously characterizedpaper. To ensure the ink droplets sufficiently overlap, a 25umdrop spacing was selected. After inkjet-printing, a low-temperature sintering step guaranteed a continuous metalconductor, providing a good percolation channel for theconduction electrons to flow. The measured and simulatedRFID tag input impedance are shown in Fig. 7. Since theconductivity of the conductive ink varies from 0.4-2.5x107

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-Simulated Resistance-Simulated Reactance

-2k------- Measured ResistanceMeasured Reactance

a -_.lI

resonator ,w feeding ine

± -nsne

|+ Lf d=25 .5mm -O14- L=26 6mm -*I h3 = 175umII4-01.1 h2 =175um*1-=8mm=h= 175um

ground plane ,

Fig. 8. Top view and side view of the proposed single-stageinkjet-printed multilayer patch resonator BPF in three layers ofpaper-based substrate.

-5

co -10

U')-sli

-20 - S21

-252 2.2 2.4 2.6 2.8

Frequency (GHz)

Fig. 9. Simulated S-parameters of the multilayer patch resonatorBPF, with center frequency at 2.4GHz and 7% bandwidth.

Siemens/m depending on the curing temperature [4], the inputresistance was slightly higher due to the additional metal lossintroduced. Overall, a very good agreement was observedover the frequency band of interest.

B. Multilayer Microwave Circuit

The evolution toward portable light-weighttelecommunication systems along with the increasing demandfor high-density packaging technologies have led to anincreasing research in multilayer technologies such as LTCCand LCP architectures. However, these applications arelimited due to the relatively expensive fabrication techniques.In this aspect, paper would be an ideal candidate as an ultra-low-cost multilayer material.

In this section, a single-stage slotted-patch band pass filterfor 2.4GHz WLAN application was designed and printedon/in paper-based substrate to demonstrate this novel solution.The configuration of the proposed structure is shown in Fig.8. The square resonator was developed from the basic half-wavelength square patch at 3.8GHz by adding a 1.75mm x8.8mm transverse cut on two sides. Transverse cuts contributeto significant additional inductance so that the operatingfrequency range was shifted by about 36%. Therefore, the patchsize was reduced significantly. The simulation result is plottedin Fig. 9. It is revealed that the insertion loss is < 0.6dB. The

Fig. 10. Photograph of the single-stage inkjet-printed multilayerpatch resonator BPF after the laminating process. The cross hairpattern is used for alignment.

bandwidth extends from 2.31GHz to 2.48GHz, which is about 700.The circuit layout on each layer was inkjet-printed

independently at the first step. After alignment, the PHIlaminator Q-247C4 was utilized for the bonding process,while 10 tons RAM force was applied under 200 'F. Thefabricated multilayer patch resonator BPF is shown in Fig. 10.This is the first time an ultra-low-cost multilayer circuit onpaper-based substrate has been demonstrated. It has to benoted that a paper-based multilayer technology would allowfor the light-weight miniaturization of "cognitive" sensingdevices, through the embedding of IC's, as well as of printedbatteries, sensors and power scavenging devices.

IV. CONCLUSION

The RF characteristics of paper-based substrates have beeninvestigated for the first time in UHF frequency band, and twobenchmarking inkjet-printed topologies have been fabricatedand tested. Our effort demonstrates that inkjet printing, as a fastprocess, can be used efficiently to print UHF and RF electronicson or in paper substrates. A compact RFID tag module, usinginductively coupling mechanism for the matching of the antennato the IC, was designed and printed on a paper-based substratefeaturing a very good performance. Paper-based multilayerstructures were also explored. These technologies could pavethe way for simple, low-cost, system-level packaging structuresfor telecommunication and sensing applications.

REFERENCES

[1] D. Thompson, G. Ponchak, M. M. Tentzeris, J. Papapolymerou,"Characterization of LCP material and transmission lines onLCP substrates from 30 to 110GHz," IEEE Trans. MicrowaveTheory & Tech., vol. 52, no. 4, pp. 1343-1352, April 2004.

[2] G. Kent, "An evanescent-mode tester for ceramic dielectricsubstrates," IEEE Trans. Microwave Theory & Tech., vol. 36,no. 10, pp. 1451-1454, October 1998.

[3] M. Janezic, "Nondestructive relative permittivity and losstangent measurements using a split-cylinder resonator," 2003Doctor ofPhilosophy Thesis, Univ. Colorado, 2003.

[4] Cabot Corporation, "Inkjet silver conductor AG-IJ-G-100-S1,"Data Sheet, October 2006.

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