188 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

RF Design for Inkjet Technology: AntennaGeometries and Layer Thickness Optimization

Vesa Pynttäri, Eerik Halonen, Hannu Sillanpää, Matti Mäntysalo, Member, IEEE, and Riku Mäkinen, Member, IEEE

Abstract—The suitability of local conductive print-layer thick-ness variation for RF applications is demonstrated on flexiblesubstrates. First, the concept is subjected to printed transmissionlines as attenuation of one- and two-layer lines is compared tolines having additional layers only on critical high-current areas.Then, two antenna types are studied by applying local additionsto the feed line and radiator with optimized print parameters foreach layer utilizing low-temperature ink enabling a variety ofsubstrate materials. For a narrow wire-type antenna, efficiencyimprovement with local thickness increase is observed both at868 MHz and 2.4 GHz, reaching the efficiency level of a fulltwo-layer antenna. For a wide monopole-type antenna at 2.4 GHz,the similar efficiency improvement up to the full two-layer levelis seen already by increasing the edge thickness on the feed line.Accordingly, the antenna type is promising for printing withsatisfactory efficiency only with one-layer print on the antennaelement. The printed antennas also show good electrical perfor-mance, with only approximately 5%–10% decrease in efficiencycompared to thick 18- m copper reference antennas.

Index Terms—Antenna measurements, flexible printed circuits,transmission lines, UHF antennas.

I. INTRODUCTION

I NKJET as a fully additive and highly flexible fabricationmethod both substrate- and layout-wise offers wide possi-

bilities for applications. These include, e.g., organic thin-filmtransistors, light-emitting diodes, solar cells, memory devices,sensors, biological tasks, and conductive structures [e.g., radiofrequency identification (RFID) tags] [1]–[3]. RF applicationsare one of the interesting areas for printed conductive structures.Transmission lines used for RF interconnections and antennasare especially good demonstrators of the technology. As printedconductor layers are very thin, in the order of micrometers, thenumber of printed layers becomes important. To minimize thelosses, suitable layer thickness should be at least in the order ofthe skin depth, i.e., the thickness in which the majority of thecurrent is concentrated. Depending on the frequency and theconductivity, several print layers may be required for the cor-responding thickness. Depending on materials used, fabricationparameters such as print resolution or sintering profile also have

Manuscript received January 04, 2012; accepted February 02, 2012. Date ofpublication February 07, 2012; date of current version March 19, 2012. Thiswork was supported by the Finnish Funding Agency for Technology and In-novation (Tekes) under Grant no. 40230/10. The work of M. Mäntysalo wassupported by the Academy of Finland under Grant no. 251882. The work of R.Mäkinen was supported by the Academy of Finland under Grant no. 125795.The authors are with the Department of Electronics, Tampere University of

Technology, 33101 Tampere, Finland (e-mail: vesa.pynttari@tut.fi).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2012.2187269

effect on high-frequency losses, which may not be visible in dcresults [4].The effect of conductive layer thickness has been previously

studied. For example, the effect for antennas has been shownwith a patch antenna [5] and a monopole [6]. An ultrawide-band (UWB) antenna on paper substrate has been printed withadequate thickness and conductivity [7]. Screen printing hasalso been applied to RFID antennas with improvement in readrange with increasing conductor thickness [8]. As inkjet tech-nology allows flexible variation between print-layer layouts, itis possible to focus the layers only on desired areas. For ex-ample, printing the conductors as gridded structures reducingthe amount of ink has been successfully tested [9], [2]. Printingadditional layers to the critical areas only, e.g., areas with highcurrent density, has also been studied with flexography printedantennas [10]. A similar idea has been used also in [2] by fab-ricating RFID tag antennas by printing only on the high-cur-rent-density areas with good results.In this letter, the effect of the conductor thickness and the

concept of concentrating ink only on critical areas are consid-ered with coplanar waveguide (CPW) lines and two types of an-tennas. The target is to reach the improved two-layer behavior inelectrical properties without printing the full two layers, whichwould result in, e.g., need of higher sintering energy, increasein process time, and limitations for substrates.First, the CPW lines are printed on a flexible substrate with

additional layers on the high-current edges to identify the effectalready on the feed line of an antenna. The line performanceis evaluated according to measured attenuation. Then, a narrowwire-type (868 MHz and 2.4 GHz) and a wide monopole-type(2.4 GHz) antenna with highly different current distributionsare studied with the method. Print parameters are optimized foreach print layer for quality and for a low-temperature ink toenable more substrate options. The effects of local additionsonly on the feed line and then on the whole antenna are studied.Finally, a comparison is made between the printed antennas anda traditional copper antenna. The suitability of antenna types forprinting with different methods is then discussed according tothe results.The antenna design and detection of high-current areas are

carried out with CST Microwave Studio (MWS) software [11].The comparison of antennas is based on efficiency measure-ments with a Satimo Starlab measurement system.

II. PRINTED TRANSMISSION LINES WITH LOCAL LAYERTHICKNESS VARIATION

Transmission lines were first fabricated to test the increasingof layer thickness only on high-current-density areas and todetect losses within the feed lines of the antennas of the study.The chosen CPW line consisting only of one conductor layer

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PYNTTÄRI et al.: RF DESIGN FOR INKJET TECHNOLOGY 189

Fig. 1. Attenuation constant extracted from the VNA measurements of theCPW lines. Inset: CPW print layouts.

is a typical choice for a printed design. Four lines were printedto a 50 m thick polyimide film, with lengths of 1, 10, 14,and 19 mm. The widths of the lines were approximately thefollowing: center line 1.0 mm, gap 190 m, and groundplane 3.0 mm. Basic one- and two-layer lines and versionswith second and third layers printed only locally on the gapedges were fabricated. CST MWS was used to define thehigh-current areas near the CPW gaps. As a result, in theorder of 160–170- m-wide additional layers besides gaps wereprinted. The basic and the local layouts are shown in the bottomof Fig. 1. For the Harima NPS-J ink used, curing conditionswere 30 min in a 250 C convection oven. Finally, the lineswere measured with a vector network analyzer (VNA) and aprobe station.For the VNA results, a multiline extraction method based

on [12] was applied to provide the attenuation constant of thelines. The attenuation up to 6 GHz is shown in Fig. 1. Dueto fabrication and measurement tolerances, some variation isseen in the results. However, the improvement between one- andtwo-layer results is apparent. In addition, the lines with a secondor third layer printed only on the high-current edges reach thelevel of the basic two-layer line. For example, at 2.4 GHz, a de-crease of 2 dB/m in attenuation is achieved by adding only lim-ited local additions on top of the one-layer line. Accordingly,the limited local printing of additional layers with CPW lines isseen as applicable and is hence recommended for reducing inkamount in printed circuit design.

III. PRINTED ANTENNA DESIGN AND FABRICATION

A. Antenna Selection and Design

Two antennas with different structure and current distribu-tion were considered to identify the effect of layer thicknessvariation for different antennas. The first antenna was a planarinverted-cone antenna (PICA) [13]. The wide monopole-typestructure designed for 2.4 GHz is shown in the top row of Fig. 2,showing high current density mostly focused on the feed line.The second selected antenna is a narrow wire-type inverted-F

Fig. 2. Basic antenna layouts, simulated current distributions, and layouts forlocally printed layers for 2.4 GHz PICA and IFA, respectively.

antenna (IFA) [14], shown in the bottom row of Fig. 2, designedalso for 2.4 GHz. Fig. 2 shows that the current distribution forthe IFA is more concentrated on the narrow radiator than in thecase of a wider PICA element. The IFA was chosen to be de-signed and fabricated also for 868-MHz frequency, as the resultsshowed more effect on the varying conductor thickness on theF-element.To achieve 50- feed line impedance, the designed width of

the CPW feed line was 3.5 mm, and the CPW gap width was100 m. The dimensions for 2.4-GHz PICA and IFA and the868-MHz IFA ground planeswere 65.0 25.0 (width length),50.0 20.0, and 121.0 24.9 mm , respectively. The totalheights of the antennas in the same order were 57.2, 27.4, and34.3 mm, respectively.

B. Antenna Fabrication

The antennas were printed with an iTi XY MDS 2.0 inkjetprinter using a Dimatix Spectra SE-128 AA printhead. Alow-temperature Harima NPS-JL nanoparticle silver ink wasused, allowing more cost-effective substrates due to lowersintering temperature (min. 120 C). Flexible 50- m polyeth-ylene napthalate (PEN) foil was selected as the substrate. Theprocess was optimized for the print equipment and materialsfor each layer, resulting in uniform layers with minimal inkconsumption. For the first layer, a method of printing only halfof the pixels of 700-dpi resolution was found suitable. As thedrop size is smaller when printed on top of existing ink, thefull 700-dpi resolution was suitable for the second layer foruniform layer. After printing, the structures were sintered for1 h in 180 C to ensure sufficient sintering with larger printedareas. Finally, SMA connectors were attached to the antennaswith electrically conductive epoxy adhesive. The referencecopper antennas were traditionally etched of 50- m PEN foilwith 18- m-thick copper layer.For reference, one- and two-layer antenna versions with

whole layers were printed. The first version of the localtwo-layer antenna included additions on the second layer onhigh-current-density areas of both the antenna element andthe feed line. In the second version for 2.4 GHz, additionswere printed only on the feed line. The local area layouts for2.4 GHz are shown in the right side of Fig. 2. For the latter localversion, only the areas on the edges of the CPW were printed.Compared to the whole antenna area, the limited local printarea is only 7% for 2.4-GHz IFA and 4% for both 2.4-GHzPICA and 868-MHz IFA. With additions only on CPW feed

190 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 3. Fabricated printed antennas. 868-MHz IFA on top, 2.4-GHz IFA onbottom left, and 2.4-GHz PICA on bottom right.

line, the corresponding values are from 2% to 3%. Examples ofthe fabricated printed antennas are shown in Fig. 3.

IV. ANTENNA MEASUREMENT RESULTS

First, the CPW gaps and the quality of the print surface wereanalyzed with a microscope. For 2.4-GHz printed antennas, themeasured CPW gap width varied between 90 and 130 m. Dueto the wide structure of 868-MHz IFA, a horizontal print di-rection was more suitable, resulting in 130- to 160- m-widegaps. For the copper references, the gap was in the order of210 m. The variation in gap widths affects the line impedance,resulting, however, only in slight variation in frequency bands.The thickness of the conductive layer was measured with an op-tical profilometer. The average measured maximum thicknesswas in the order of 1.0 and 1.7 m for one- and two-layer struc-tures, respectively. The values were slightly higher on the edgesof the conductors and slightly lower values at the center of largerprinted areas.

A. 2.4-GHz PICA Results

For the fabricated 2.4-GHz PICAs, the 10-dB return loss (RL)bandwidth is satisfied for all antennas above 2.4 GHz. The dcsheet resistance for one-layer is in the order of 110 m , and24 m for two-layer. The results suggest in the order of2.5e7 S/m conductivity for the two-layer conductor. As a result,the skin depth is 2.1 m at 2.4 GHz. Accordingly the maximumthickness is below the value of skin depth, resulting in expectedefficiency improvement with layer thickness increase and localthickness additions.The measured total efficiency is shown in Fig. 4. Above

2.5 GHz, the two-layer and both local print versions have sim-ilar efficiency. Up to 3 GHz, the difference to one-layer versionis in the order of 5%. At 2.6 GHz with RL better than 15 dBfor all antennas, the efficiency for copper reference is 86%,81% for printed two-layer and both local versions, and 76%for one-layer print. According to this, similar improvementcompared to one-layer print is achieved by either printing thesecond layer totally, printing the layer only on local areas onboth the antenna and CPW line, or printing the additions onlyto the CPW line. The latter is obviously the suggested optionfor fabrication in the case of wide monopole-type antennas.Finally as the copper reference antenna is compared to

improved printed antennas, the difference in efficiency is only

Fig. 4. Measured total efficiency of PICA for 2.4-GHz frequency.

in the order of 5%–10% at frequencies with similar RL value,showing the good performance achieved with printed widemonopoles. The comparison also shows that the efficiencydifference between one-layer printed antenna and the copperantenna can be notably decreased by printing an additionallayer only to a limited area near the CPW feed line, withoutprinting additional layers to the antenna element itself.

B. 2.4-GHz IFA Results

For the 2.4-GHz IFA, the measured resonant frequency is ata slightly lower frequency than designed. According to sim-ulations, the addition of an SMA connector is the reason forthis, as the fabricated CPW lines are not perfectly 50 . How-ever, 10-dB RL condition is satisfied from 2.0 to 2.4 GHz forall antennas. In sheet resistance results, there is more varia-tion compared to PICA results. The one-layer average resultis 174 m . The result for the version with local additionsonly on the CPW line is 69 m , and 222 m for the ver-sion with local additions on both the antenna and CPW line. Thesheet resistance of the two-layer print is 20 m .The measured total efficiency values are shown in Fig. 5.

Again, the efficiency of the one-layer version is the lowest. Thetwo-layer and local two-layer results are at the same level in themiddle of the band. Otherwise, the local two-layer antenna iseven better than the two-layer version, although the differenceis small. The version with a second layer only on the feed-linearea has also improved efficiency compared to one-layer print,but is, however, at a slightly lower level than the full two-layerlocal version. The values at 2.2 GHz with similar RL value are81% for copper, 73% for the local two-layer, 73% for two-layerversion, 71% for the local feed-line two-layer version, and fi-nally 69% for one-layer print.The results show, that the difference between copper refer-

ence and printed antennas is larger thanwith the widemonopole.Advance with printed antennas is again achieved with only localadditions on the second print layer. The local two-layer ver-sion is even slightly better than the whole two-layer version,although the bottom layer of the local version is measured tohave unexpectedly poor sheet resistance. The addition only onthe CPW line is not adequate to reach the whole local two-layerefficiency, although the sheet resistance for the first layer is in

PYNTTÄRI et al.: RF DESIGN FOR INKJET TECHNOLOGY 191

Fig. 5. Measured total efficiency of IFA for 2.4-GHz frequency.

Fig. 6. Measured total efficiency of IFA for 868-MHz frequency.

a particularly good level for a one-layer print. An inferior valuewould increase the difference.

C. 868-MHz IFA Results

For the 868-MHz antennas, the level of RL is lower, witha 4-dB result for the copper reference and 6 dB for printedantennas. The resonant frequencies, however, remain at830–840 MHz with all antennas. The dc sheet resistance forone-layer is 118 m , and 36 m for two-layer. In Fig. 6is shown the measured total efficiency of the antennas. Themeasured maximum efficiencies are 45%, 49%, 49%, and 57%for one-layer, two-layer, local two-layer, and reference, re-spectively. Again, the local version has the same performanceas the two-layer version. The relative efficiency differencebetween printed antennas and the copper reference is increasedcompared to 2.4 GHz due to the increased skin depth value atlower frequency compared to the thickness.

V. CONCLUSION

Printing additional layers only on critical areas was shownto be effective in terms of attenuation loss of transmission lines

and total efficiency of antennas. The result of two printed layerswas achieved by printing the second layer only on high-cur-rent-density areas. For printing, the process was adaptedoptimally for multiple layers with minimum ink in the firstlayer. For CPW lines, about 2 dB/m decrease in attenuationwas seen with limited printing, showing a promising optionfor decreasing loss without highly increased ink usage. Forantennas, the loss within the CPW feed line was also seen asimportant. For a wide monopole with only low current densityin the radiator, the effect of the additions only at the feed lineproduced the efficiency level of two-layer print. For a narrowwire-type antenna with high current density located also onthe antenna element, the local additions were required on theantenna element too for the efficiency level of two-layer print.Hence, the antenna geometry selection according to availableprocess steps and application requirements is important. Also,with local layer additions to antennas, the efficiency differencebetween the copper reference antenna and the one-layer printedantenna was clearly decreased.

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