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Inkjet printing of UHF antennas on corrugated cardboards for packaging applications
Applied Surface Science 332 (2015) 500–506
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Inkjet printing of UHF antennas on corrugated cardboards for packaging applications Enrico Sowade a,∗,1 , Frank Göthel a,1 , Ralf Zichner b,1 , Reinhard R. Baumann a,b a b
Digital Printing and Imaging Technology, Technische Universität Chemnitz, Chemnitz, Germany Department Printed Functionalities, Fraunhofer Institute for Electronic Nano Systems (ENAS), Chemnitz, Germany
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Article history: Received 10 November 2014 Received in revised form 15 January 2015 Accepted 18 January 2015 Available online 28 January 2015 Keywords: Printed electronics Inkjet printing UV-curable ink Ultra-high frequency (UHF) antenna Passive radio-frequency identification (RFID)
a b s t r a c t In this study, a method based on inkjet printing has been established to develop UHF antennas on a corrugated cardboard for packaging applications. The use of such a standardized, paper-based packaging substrate as material for printing electronics is challenging in terms of its high surface roughness and high ink absorption rate, especially when depositing very thin films with inkjet printing technology. However, we could obtain well-defined silver layers on the cardboard substrates due to a primer layer approach. The primer layer is based on a UV-curable ink formulation and deposited as well as the silver ink with inkjet printing technology. Industrial relevant printheads were chosen for the deposition of the materials. The usage of inkjet printing allows highest flexibility in terms of pattern design. The primer layer was proven to optimize the surface characteristics of the substrate, mainly reducing the surface roughness and water absorptiveness. Thanks to the primer layer approach, ultra-high-frequency (UHF) radio-frequency identification (RFID) antennas were deposited by inkjet printing on the corrugated cardboards. Along with the characterization and interpretation of electrical properties of the established conductive antenna patterns, the performance of the printed antennas were analyzed in detail by measuring the scattering parameter S11 and the antenna gain. © 2015 Elsevier B.V. All rights reserved.
1. Introduction and state-of-the-art Today, products of printing are omnipresent in everyday life, e.g. the packaging of consumer goods. Some product information is printed directly on the good after its manufacturing, e.g. the best before date and similar information. Digital printing methods such as inkjet printing are well-established for these purposes. However, during the last decades the field of printed electronics has attracted growing interest. In this sense, inkjet printing could not only be used to print the best before date, but also to print functional features on packages, such as RFID antennas. As printing technologies are considered to be a cost-efficient production process allowing high through-put (e.g. Roll-to-Roll (R2R) manufacturing) and large-area manufacturing, they could also become a promising method for the mass production of various electronic components in future. In present, so-called smart labels including an RFID chip and antenna are already used in
∗ Corresponding author at: Technische Universität Chemnitz, Digital Printing and Imaging Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany. E-mail address: [email protected] (E. Sowade). 1 The authors contributed equally to the presented research work. http://dx.doi.org/10.1016/j.apsusc.2015.01.113 0169-4332/© 2015 Elsevier B.V. All rights reserved.
the industry for logistical issues. However, the costs for such passive labels without an additional battery are still too high due to the production via micro fabrication processes, e.g. photolithography or wet etching. Such subtractive manufacturing methods have a high material wastage as well as high process restrictions under clean room conditions. Besides that, an additional step to implement the tags on the final product is required. For that reason, the aim is to combine the cost-efficient inkjet printing technology and paper, the cheapest and environmentally friendliest substrate material, within one process to develop a functional antenna for RFID applications on a packaging material. With respect to printed electronics, the usage of inkjet printing as well as paper has a lot of challenges and restrictions, particularly in combination with each other. In the field of printed electronics usually non-absorbent substrates, such as glass, plastics (e.g. PET, PEN, etc.) and silicon exhibiting smooth and homogeneous surfaces are preferred. Such substrates are in general much more expensive than conventional paper. In fact, paper is a very complex material with different levels of roughness, porosity and absorption capacity. But it has been also considered as substrate for printed electronics by many researchers due to its lower costs and recyclability. Recently, there is a lot of scientific research focusing on the potential use of printing technologies for low
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cost mass production of especially UHF antennas on different substrates. Traditional printing technologies such as screen printing and gravure printing have been proven to be suitable technologies for a wide range of substrates, including paper-based substrates such as corrugated cardboards [1–3]. Research groups investigated also the potential of different paper substrates for printed electronics based on thin films deposited via inkjet technology [4,5]. They proved that certain paper materials are suitable for the inkjet deposition of functional layers, e.g. special inkjet papers equipped with polymer coatings ensuring low roughness and optimized solvent absorption. Denneulin et al. and Puukko et al. demonstrated the optimization of paper substrate surfaces to make it compatible with the deposition of functional layers for printed electronics. The optimization is based on self-made UV-curable inkjet ink formulations to modify different paper types for inkjet printing [12,13]. Denneulin et al. presented the high potential of a UV primer layer for deposited carbon nanotube (CNT) patterns directly on paper-based substrates. We use the same approach, but showing the feasibility of printing silver-based UHF antennas with adapted scattering parameters and industrial relevant inkjet printheads. To the best of our knowledge, the first inkjet-printed UHF antenna on paper substrate was reported in 2007 by Yang et al. [6]. Since then, several scientific papers have been published, e.g. by Amin et al. or Rida et al., focusing on optimized antenna designs regarding the dielectric properties of the used paper substrates and the characteristics of the antenna [7–11]. Next to paper substrates, many publications report about the inkjet printing of antennas on nonabsorbent substrates such as FR4, glass, kapton or PET [14–17]. Virtanen et al. [18] deposited a similar silver ink as used in this contribution on cardboard substrates. They discuss in detail the usage of wood-based substrates and the required adjustments for the inkjet process. The high ink absorptiveness of the substrates is compensated by printing several layers (20 layers for the cardboard substrate) applying a laboratory printhead with 10 pL nominal drop volume [18]. The drawback of this approach is the high silver material consumption and the very long processing time, especially when sintering of each individual silver layer is performed for 60 min. In contrast, we demonstrate a very fast inkjet deposition based on only one silver layer and a UV-curable primer layer. That will limit the consumption of the expensive silver material and the fast deposition facilitates also roll-to-roll processing.
2. Materials and methods The paper-based corrugated cardboard (1.30 B, Dunapeck Spremberg GmbH) employed in this work is a standardized packaging material for the transportation of goods worldwide. The substrate is composed of three different parts: (i) kraft liner top board, (ii) single flute middle part and (iii) test liner bottom board. The flute has a thickness of about 3.2 mm and is made of test liner material. The kraft liner consists of unbleached kraft pulp (sulphate wood pulp) and has a higher quality than the test liner, which is made from recycled paper. For the conductive layer, a commercially available silver ink (SunTronic EMD5603; SunChemical Corporation) was applied. It is a solvent-based nanoparticle ink optimized for jetting by various piezoelectric inkjet printheads. According to the specification of the manufacturer, the silver particle size is in the range of 30–50 nm dispersed in a mixture of ethylene glycol, ethanol and glycerol. The content of silver is about 20 wt%. The ink has a viscosity of 10–13 mPa·s at 25 ◦ C and a surface tension of 27–31 mN/m. For the primer layer, a UV-curable ink (V-Photon Clear NonWet; Tritron GmbH) was employed. It is based on acrylates and allows
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chemical curing by UV radiation in the range of 385–395 nm. Thus, the transfer from a wet layer to a solid layer due to cross-linking and polymerization is photo-chemically induced. The viscosity of the ink is in the range of 16–19 mPa s at 25 ◦ C and the surface tension in the range of 31–33 mN/m at 25 ◦ C (specification of the manufacturer). Both of the inks were deposited with drop-on-demand inkjet systems (DMP 2831, DMP 3000; Dimatix Fujifilm Inc.). The printers operate with piezoelectric printheads and are designed and optimized to print various functional materials. Different printheads were used in the research, depending on the selected ink as well as on the printed pattern dimensions. First investigations have been made for all inks with the Dimatix Material Cartridges (DMC) 11610 consisting of a fluid bag and a laboratory printhead. The printhead is equipped with 16 nozzles arranged in a single line generating drops with a nominal volume of 10 pl. Two different industrial printheads with 128 nozzles arranged in a single line have been used for larger pattern dimensions. The SE3 printhead with a nominal drop volume of 35 pL was employed for printing the silver ink EMD5603 and the SX3 printhead with a nominal drop volume of 8 pL for depositing the UV-curable ink. To functionalize the printed layers, different post-treatment processes were carried out. UV-curing (SubZero 085 ES System, Integration Technology Ltd.) was used to polymerize the UV inks. A standard convection oven (Carbolite Type 200, Carbolite Ltd.) was employed for the drying and sintering of the deposited silver ink at 150 ◦ C for 30 min. CST Microwave Studio was used for the simulation of the RFID antenna parameters. A 3D-model of the antenna design (as shown in Figs. 9 and 11) was created digitally. Next to the antenna geometry following parameters of the silver layer were assumed: Electrical conductivity of 5 × 106 S/m and layer thickness of 2 m. The simulation was done with a standard 50 port impedance to show the antenna functionality independent of any specific RFID chip impedance. Antenna matching for specific RFID chip impedance can be done easily by slight antenna design modification. The antenna characterization was carried out in an anechoic chamber. Relevant antenna parameters such as the scattering parameter S11 were measured with the network analyzer ZVL6 (Rohde & Schwarz). The antenna gain was determined by a spectrum analysis of a communication link between the device under test and a calibrated patch antenna. The antenna was contacted electrically with a conductive adhesive tape (3 M Tape 9703) to a coaxial cable. Scanning electron microscopy (SEM, TM-1000, Hitachi) was performed to analyze the surface of the cardboard substrate. Surface profilometry measurements were done with a Dektak 150 system (Veeco). The absorptiveness of the cardboard substrate was determined with the method according to ISO 535 (Cobb 60 s).
3. Results and discussion 3.1. Corrugated cardboard substrate Only the bottom board (kraft liner) of the corrugated cardboard was used for the deposition of the silver ink. Fig. 1 shows the SEM image of the surface of the bottom board. The difference in size, density and arrangement of the cellulose fibers at the surface of the substrate causes a high arithmetic average roughness of about 9.7 m ± 0.4 m. The water absorptiveness was determined using a Cobb tester according to ISO 535 for 60 s. The obtained water absorptiveness was <33 g/m2 which corresponds approximately to the absorptiveness of standard office papers.
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Fig. 1. SEM image of the kraft liner surface of the cardboard substrate showing the fiber sizes and their arrangement.
Fig. 2. Microscopic image of the edge of a printed and sintered silver pattern; area (a) indicates the printed silver layer and (b) the bare surface of the cardboard substrate.
3.2. Development of primer layer The silver ink was printed on cardboard paper in patterns of 5 mm × 5 mm. By changing the resolution of the printer, different amounts of drops per area can be obtained. The patterns were printed with a resolution of 1270 dpi up to 5080 dpi. Due to the high absorption capacity of the substrate and the high surface roughness, the low viscous silver ink penetrated into the paper for all print resolution. No conductivity was obtained after the thermal posttreatment which was applied to sinter the silver nanoparticles. The high absorption capacity and surface roughness avoid the formation of a conductive network of silver particles. As the microscopic image in Fig. 2 shows, a homogenous silver film has not formed on top of the substrate. The edges of the deposited silver are marked with the dotted line. Area (a) indicates the printed silver layer and (b) the non-coated cardboard surface. Obviously, the edges of the silver pattern are very irregular because of the inhomogeneous ink
Fig. 4. Photographic images of inkjet-printed UV ink Tritron V-Photon NonWet on cardboard with (a) 5080 dpi, (b) 2540 dpi, (c) 1693 dpi and (d) 1270 dpi print resolution, the red rectangle indicates the digital pattern size. (For interpretation of the color information in this figure as well as for the following figures, the reader is referred to the colored web version of the article.)
penetration into and along the paper fibers of the surface. Even increasing the number of stacked layers at 5080 dpi print resolution up to five could not improve the electrical properties. No electrical conductivity could be measured as a result of the high roughness and absorption capacity. In consequence, a primer layer based on the concept of Denneulin [12] was developed. The concept of the primer layer is shown in Fig. 3. An ideal layer formation can be obtained on polymer substrates which have a smooth and non-absorbent surface. A primer layer is required to improve the bad layer formation on cardboard surfaces. The primer layer can ensure a proper layer formation of the silver ink printed on top of it since it reduces the absorptiveness and the roughness of the substrate. A UV-curable ink formulation was selected as primer material to allow the deposition of thick layers as a cardboard roughness of about 9.7 m ± 0.4 m needs to be compensated. Additionally, the UV-curing step can be adjusted to control the ink penetration. The Tritron V-Photon Clear NonWet ink was deposited on the cardboard using the DMP2831 in patterns of 5 mm × 5 mm. Fig. 4 shows the results as a function of print resolution. Following print resolutions were used: Fig. 4 (a) 5080 dpi, (b) 2540 dpi, (c) 1693 dpi and (d) 1270 dpi. In case of 5080 dpi, the layers on top of the cardboard substrate are clearly visible in form of a glossy coating with high thickness. Around the glossy rectangular layers one can see a lot of ink material with low edge sharpness. The reasons are the high absorptiveness of the cardboard and comparable slow printing process with a long time between layer deposition and layer curing. Faster printing and faster curing as usual for industrial print system will allow sharp patterns without excessive material absorption. Decreasing the print resolutions from Fig. 4 (a) to (d) results in thinner layers where most of the material is absorbed by the substrate. To improve the primer layer, a second layer of the ink was printed on top of the already cured UV layer (cured for 9 s using the SubZero 085 ES UV system). Fig. 5 shows the resulting patterns when a second layer with print resolutions of 5080 dpi, 2540 dpi, 1693 dpi and 1270 dpi was printed on the first layer. The first
Fig. 3. Scheme of ink layer formation on different surfaces [based on 12].
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Fig. 5. Photographic images of two layers of inkjet-printed UV ink Tritron V-Photon NonWet on cardboard with different print resolutions for the first and second layer.
column results in irregular pattern formation with sizes much bigger than 5 mm × 5 mm. Additionally, a lot of material is absorbed by the substrate. The third and the fourth columns show partly developed shiny layers, at least in case of 2540 dpi for the first layer. The UV ink was completely absorbed at lower print resolutions. The best result was obtained at a print resolution of 2540 dpi for both layers. It results in a squared layer with reasonable high edge sharpness and comparable low material absorption. The second layer was cured using the same conditions applied for the first layer. The thickness of the developed primer coating consisting of two inkjet-printed UV Tritron V-Photon NonWet layers with 2540 dpi print resolution was measured to about 15 m with an average roughness of 200–300 nm. Fig. 6 shows a comparison of the surface profiles of the bare cardboard kraft liner surface and primer layer surface on top of the kraft liner. The vertical movement of the styles of the Dektak profiler is shown as a function of the measurement length. Obviously, the roughness was remarkably improved. All the peaks of up to 15 m were covered by the printed primer layer which forms a homogeneous and smooth surface.
Fig. 6. Surface profile of kraft liner with and without the printed primer layer.
3.3. Improvement of the primer layer Following, the silver ink was deposited on top of the primercoated kraft liner using inkjet printing. A print resolution of 1270 dpi was used. The pattern was defined as square with the size of 4 mm × 4 mm. Fig. 7 shows the deposited layer on top of the UV ink. It can be seen, that the silver ink merges as agglomerated drop close to the center of the square due to dewetting effects. The primer surface needs to be modified in order to enable the deposition of a homogeneous silver layer on top. To investigate the dewetting, the surface energy of the primer layer was determined as a function of UV treatment of the deposited primer layer. The static sessile drop method was applied to measure the contact angle of different solvents with the primer layer (MobileDrop system, Krüss GmbH). Based on Owen–Wendth–Rabel–Kaelble, the surface energy was calculated. Fig. 8 shows the result of the surface energy as a function of treatment. The primer layer without any treatment has a surface energy of 28.9 mN/m. The polar part has a very low value of about 1.9 mN/m. Additional application of UV irradiation which was used for the curing process of the UV ink anyway, results in an increase of surface
Fig. 7. Agglomerated silver layer on top of primer-coated cardboard.
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Fig. 10. Sheet resistance and layer thickness of EMD5603 printed on top of the primer layer as a function of print resolution.
Fig. 8. Surface energy as a function of surface modification of the UV-cured primer layer (Tritron V-Photon Clear NonWet).
energy. With a UV irradiation of 6 s, the surface energy has been increased to about 35 mN/m with a polar part of approximately 4 mN/m. The obtained surface energy with 6 s additional UV treatment improved the wetting of the silver on top of the primer remarkably. Continuous silver layers were obtained on the UV primer with reasonable conductivity. 3.4. RFID antenna design and manufacturing A special UHF dipole antenna design adapted to the cardboard substrate was developed to validate the applicability for RFID. The designed antenna including the primer layer is shown in Fig. 9. As explained before, the primer layer was printed two times each with a resolution of 2540 dpi. The printing of the UV layers was performed with the DMP3000 equipped with a Fujifilm Dimatix SX3 printhead. The area size of the first UV layer was increased by 1 mm and the size of the second UV layer by 0.5 mm with regard to the silver antenna layer. Each layer was cured by UV irradiation for 6 s. Prior to printing the silver layer, the surface of the primer layer was activated by additional 6 s UV treatment to improve the wetting behavior as explained before. Printing of the silver layer was carried with a Fujifilm SE3 printhead using a print resolution of (i) 847 dpi, (ii) 1016 dpi and (iii) 1270 dpi. The measured sheet resistances of the printed patterns as a function of print resolution and layer thickness are depicted in Fig. 10. The layer thickness was
Fig. 9. Designed layer stack consisting of UV primer layers and the UHF silver antenna pattern.
measured with the Dektak profiler at a printed line with 500 m width. As visible, the sheet resistance decreases approximately linear with the print resolution. Also the layer thickness increased as expected with increasing print resolution. The print resolution will have a significant influence on the performance of the designed UHF antenna. Not only the conductivity influences the antenna parameters, also the geometrical accuracy of the antenna design is of importance. Fig. 11 shows the designed antenna in detail including the most important dimensions. The inset section highlights one of the most important geometrical parameters for the antenna performance: the width of the gap. Here, the width of the gap is marked as (a) if the gap length position is perpendicular to the printing direction and (b) if the gap length is along the printing direction. The digital gap width for (a) is defined as 500 m and the digital gap width for (b) as 650 m. The patterns were printed with the before mentioned print resolutions and the gap widths of the prints were measured with a light microscope. Fig. 12 shows the measured gap widths as a function of printing direction and print resolution. The digital gap widths are indicated with the dotted line. It can be seen that perpendicular to the printing direction the gap widths are almost in the range of the digital dimension. However, the gap width in printing direction is for all print resolutions
Fig. 11. Digital antenna pattern, inset marks the gap width positions (a) perpendicular to printing direction and (b) in printing direction.
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Fig. 12. Gap width of the inkjet-printed UHF antenna on top of the primer layers as a function of print resolution and printing direction.
Fig. 13. Inkjet-printed UHF antenna on cardboard substrate in anechoic chamber for measurement of scattering parameters.
smaller than the digital size. The higher the print resolution, the larger the deviation to the designed digital gap width. Therefore, with the lowest print resolution the highest geometrical accuracy with respect to the designed antenna was obtained. The manufactured UHF antennas were characterized concerning their scattering performance in an anechoic measurement chamber as depicted in Fig. 13. The connection was established with adhesive tape and a coaxial cable to the network analyzer. The scattering parameter S11 was determined during the experiment allowing the deduction of the reflective properties of the UHF antenna. In general, a S11 value in the range of −15 dB and −30 dB normally represents a good adapted antenna qualified for RFID applications. A wave impedance value of 50 has been set for the measurement. The measurement results are plotted in Fig. 14 for each antenna
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Fig. 15. Antenna gain of inkjet-printed antennas as a function of print resolution.
print resolution. Positively, the measured scattering parameter in Fig. 14 (A) show a higher bandwidth compared to the simulated results in Fig. 14 (B). The simulated graph shows a wavy curve next to the peak position due to interactions between the dipol and the director. It can be seen that all antennas printed on the primer layer on the cardboard kraft liner are functional. They have a resonance frequency in the range of 0.988 GHz. Interestingly, the lowest S11 value of about −32 dB was obtained for the lowest print resolution (847 dpi) where the sheet resistance was very high with about 1.78 ± 0.53 /sq. Obviously, the geometrical accuracy has a very big influence since a print resolution of 847 dpi was close to the desired antenna design. With increasing print resolution, the antenna performance decreases as indicated by the S11 parameter. The S11 parameter increases to about −9.5 dB for the antenna printed with 1270 dpi. The performance of the antenna with 1016 dpi is even worse, most probably due the geometrical deviation of the gap width perpendicular to printing direction. This experiment shows that one of the most critical factors for the antenna functionality is the compliance of the antenna geometry. Deviations from the antenna design seem to influence more prominent the antenna performance than the conductivity. Next to the scattering parameter the antenna gain was determined. Fig. 15 shows the antenna gain at 50 as a function of frequency of the inkjet-printed antennas with print resolutions of 847 dpi, 1016 dpi and 1270 dpi. The simulation is shown in the graph as well. There are some deviations comparing the simulation and measured data, especially at lower and higher frequencies. But overall, both the simulation data and the measurement data are in the same range. At a frequency of 0.9885 GHz the antenna
Fig. 14. (A) Measured scattering parameter S11 of inkjet-printed antennas as a function of print resolution and (B) simulated scattering parameter S11 (CST Microwave Studio).
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gain varies between 2 dBi and 2.3 dBi depending on the print resolution of the antenna. The lowest gain was determined for the antenna with a print resolution of 1016 dpi—the same print resolution where the lowest scattering performance was measured as shown in Fig. 14. The obtained antenna gain in the range of 2 dBi is typically comparable to aluminum etched planar RFID transponder dipoles. Thus, we can demonstrate that the inkjet-printed antennas on the cardboard substrates are functional with comparable gain of aluminum-etched dipoles. Therefore, they could potentially operate in RFID applications. The highest antenna gain of about 3.3 dBi was obtained at 1.106 GHz for a print resolution of 1270 dpi. This print resolution results in the highest layer thickness and the lowest electrical resistance compared to 847 dpi and 1016 dpi as shown in Fig. 10. 4. Summary and conclusion The focus of the contribution was set on the development of UHF antennas on corrugated cardboard substrates using inkjet printing. Industrial relevant printheads were chosen for inkjet printing of the different functional materials. In order to facilitate the usage of cardboard substrates for printed electronics, a primer layer was deposited in patterns and optimized towards a homogeneous film formation of the silver layer on top of it. Without this primer layer, we could not obtain a conductive film of silver ink—even with multiple printed layers at high print resolution. A UV-curable, commercially available ink formulation was applied for the primer layer. Deposition of two layers of the UV ink formulation enables the formation of a smooth surface and thus coverage of all the peaks and valleys of the rough cardboard surface. The developed UV ink primer layer was improved concerning its surface energy by additional UV treatment. A special antenna design was developed and printed via inkjet technology on the primer layers. The morphology of the printed silver layers was characterized focusing on the deviations to the digital antenna design. Additionally, the layer thickness was determined as a function of print resolution. The performance of the printed antennas was analyzed in an anechoic chamber based on the scattering parameter S11 and the antenna gain. All of the printed antennas were functional. The best UHF antenna regarding the scattering parameter S11 was obtained with a comparable low print resolution of 847 dpi. Antennas with a print resolution of 1270 dpi resulted in the highest antenna gain of about 3.3 dBi at a frequency of 1.106 GHz. The antenna gain at 0.9885 GHz is in the range of 2 dBi to 2.3 dBi. There is only a minor difference between 1270 dpi and 847 dpi. The low print resolution is advantageous concerning the costs, since a lower print resolution refers to less material consumption. Our results indicate that the geometrical accuracy of the antenna seems to be more important than the conductivity of the antenna. Antennas with a S11 value of −32 dB and antenna gains in the range of 2 dBi at the resonance frequency of 0.988 GHz were manufactured successfully on corrugated cardboards using exclusively inkjet printing. This can be considered as important step towards potential applications for the packaging industry.
Acknowledgements Ralf Zichner was financially supported by the SAB project WeFuFlex. Kalyan Yoti Mitra is acknowledged for his support for preparing the SE3 printhead and his support during printing of the silver layer at the DMP3000. We thank Dr. Siegrun Ulbricht and Mrs. Karin Förster for their help with the Cobb tester. Enrico Sowade was financially supported by the European Commission within the Framework FP7-ICT (grant agreement number 287682, project acronym TDK4PE). References [1] T. Muck, M. Staresinic, D.G. Svetec, M. Pivar, U. Kavcic, M. Staresinic, Printed HF and UHF RFID antenna directly on cardboard and recycled paper, LOPE-C (2012) 398–401. ¨ [2] S. Merilampi, L. Ukkonen, L. Sydanheimo, P. Ruuskanen, M. Kivikoski, Analysis of silver ink bow-Tie RFID tag antennas printed on paper substrates, Int. J. Antennas Propag. (2007) 1–9. [3] M. Rebros, E. Hrehorova, B. Bazuin, Rotogravure printed UHF RFID antennae directly on packaging materials, in: TAGA 60th Annual Technical Conference (2008) 292–304. [4] T. Öhlund, J. Örtegren, S. Forsberg, H.-E. Nilsson, Paper surfaces for metal nanoparticle inkjet printing, Appl. Surf. Sci. 259 (2012) 731–739. [5] L. Xie, M. Mäntysalo, A.L. Cabezas, Y. Feng, F. Jonsson, L.-R. Zheng, Electrical performance and reliability evaluation of inkjet-printed Ag interconnections on paper substrates, Mater. Lett. 88 (2012) 68–72. [6] L. Yang, A. Rida, R. Vyas, M.M. Tentzeris, RFID Tag and RF structures on a paper substrate using inkjet-printing technology, IEEE Trans. Microwave Theory Tech. 55 (2007) 2894–2901. [7] Y. Amin, S. Prokkola, B. Shao, J. Hållstedt, H. Tenhunen, L. Zheng, Inkjet Printed Paper Based Quadrate Bowtie Antennas For UHF RFID Tags, in: 11th International Conference in Advanced Communication Technology (2009) 109–112. [8] M.M. Tentzeris, Inkjet-printed paper-based RFID and nanotechnology-based ultrasensitive sensors: The ‘Green’ Ultimate Solution for an ever improving Life Quality and Safety!?, in: IEEE Conference on Radio and Wireless Symposium (2010) 120–123. [9] A. Rida, L. Yang, R. Vyas, M.M. Tentzeris, Conductive inkjet-printed antennas on flexible low-cost paper-based substrates for RFID and WSN applications, IEEE Antennas Propag. Mag. 51 (2009) 13–23. [10] G. Shaker, S. Safavi-naeini, N. Sangary, A.P. Setup, Inkjet printing of Ultrawideband (UWB) antennas on paper-based substrates, Antennas Wireless Propag. Lett. 10 (2011) 111–114. [11] M. Marroncelli, D. Trinchero, Concealable, low-cost paper-printed antennas for WISP-based RFIDs, in: IEEE International Conference on RFID (2011) 6–10. [12] A. Denneulin, J. Bras, A. Blayo, C. Neuman, Substrate pre-treatment of flexible material for printed electronics with carbon nanotube based ink, Appl. Surf. Sci. 257 (2011) 3645–3651. [13] P. Puukko, A. Ilmonen, T. Lamminmäki, S. Sundqvist, V.T.T. Technical, UV-inkjet Ink Penetration and Its Effect on Print Quality Formation and Drying, NIP 25: International Conference on Digital Printing Technologies and Digital Fabrication (2009) 566–569. [14] Y. Al-Naiemy, T.A. Elwi, H.R. Khaleel, A. Hussain Al-Rizzo, Systematic approach for the design, fabrication, and testing of microstrip antennas using inkjet printing technology, ISRN Commun. Network. (2012) 132465. [15] V.K. Palukuru, A. Pekonen, V. Pynttäri, R. Mäkinen, J. Hagberg, Jantunen Heli, An inkjet-printed inverted-F antenna for 2.4-GHz wrist applications, Microwave Opt. Technol. Lett. 51 (2009) 2936–2938. [16] J. Virtanen, J. Virkki, A.Z. Elsherbeni, L. Sydänheimo, L. Ukkonen, A selective ink deposition method for the cost-performance optimization of inkjet-printed UHF RFID tag antennas, Int. J. Antennas Propag. (2012) 801014. [17] M. Mäntysalo, P. Mansikkamäki, An inkjet-deposeted antenna for 2.4 GHz applications, Int. J. Electron. Commun. 63 (2009) 31–35. [18] J. Virtanen, J. Virkki, L. Ukkonen, L. Sydänheimo, Inkjet-printed UHF RFID tags on renewable materials, Adv. Internet Things 2 (2012) 79–85.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Inkjet printing of UHF antennas on corrugated cardboards for packaging applications Enrico Sowade a,∗,1 , Frank Göthel a,1 , Ralf Zichner b,1 , Reinhard R. Baumann a,b a b
Digital Printing and Imaging Technology, Technische Universität Chemnitz, Chemnitz, Germany Department Printed Functionalities, Fraunhofer Institute for Electronic Nano Systems (ENAS), Chemnitz, Germany
a r t i c l e
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Article history: Received 10 November 2014 Received in revised form 15 January 2015 Accepted 18 January 2015 Available online 28 January 2015 Keywords: Printed electronics Inkjet printing UV-curable ink Ultra-high frequency (UHF) antenna Passive radio-frequency identification (RFID)
a b s t r a c t In this study, a method based on inkjet printing has been established to develop UHF antennas on a corrugated cardboard for packaging applications. The use of such a standardized, paper-based packaging substrate as material for printing electronics is challenging in terms of its high surface roughness and high ink absorption rate, especially when depositing very thin films with inkjet printing technology. However, we could obtain well-defined silver layers on the cardboard substrates due to a primer layer approach. The primer layer is based on a UV-curable ink formulation and deposited as well as the silver ink with inkjet printing technology. Industrial relevant printheads were chosen for the deposition of the materials. The usage of inkjet printing allows highest flexibility in terms of pattern design. The primer layer was proven to optimize the surface characteristics of the substrate, mainly reducing the surface roughness and water absorptiveness. Thanks to the primer layer approach, ultra-high-frequency (UHF) radio-frequency identification (RFID) antennas were deposited by inkjet printing on the corrugated cardboards. Along with the characterization and interpretation of electrical properties of the established conductive antenna patterns, the performance of the printed antennas were analyzed in detail by measuring the scattering parameter S11 and the antenna gain. © 2015 Elsevier B.V. All rights reserved.
1. Introduction and state-of-the-art Today, products of printing are omnipresent in everyday life, e.g. the packaging of consumer goods. Some product information is printed directly on the good after its manufacturing, e.g. the best before date and similar information. Digital printing methods such as inkjet printing are well-established for these purposes. However, during the last decades the field of printed electronics has attracted growing interest. In this sense, inkjet printing could not only be used to print the best before date, but also to print functional features on packages, such as RFID antennas. As printing technologies are considered to be a cost-efficient production process allowing high through-put (e.g. Roll-to-Roll (R2R) manufacturing) and large-area manufacturing, they could also become a promising method for the mass production of various electronic components in future. In present, so-called smart labels including an RFID chip and antenna are already used in
∗ Corresponding author at: Technische Universität Chemnitz, Digital Printing and Imaging Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany. E-mail address: [email protected] (E. Sowade). 1 The authors contributed equally to the presented research work. http://dx.doi.org/10.1016/j.apsusc.2015.01.113 0169-4332/© 2015 Elsevier B.V. All rights reserved.
the industry for logistical issues. However, the costs for such passive labels without an additional battery are still too high due to the production via micro fabrication processes, e.g. photolithography or wet etching. Such subtractive manufacturing methods have a high material wastage as well as high process restrictions under clean room conditions. Besides that, an additional step to implement the tags on the final product is required. For that reason, the aim is to combine the cost-efficient inkjet printing technology and paper, the cheapest and environmentally friendliest substrate material, within one process to develop a functional antenna for RFID applications on a packaging material. With respect to printed electronics, the usage of inkjet printing as well as paper has a lot of challenges and restrictions, particularly in combination with each other. In the field of printed electronics usually non-absorbent substrates, such as glass, plastics (e.g. PET, PEN, etc.) and silicon exhibiting smooth and homogeneous surfaces are preferred. Such substrates are in general much more expensive than conventional paper. In fact, paper is a very complex material with different levels of roughness, porosity and absorption capacity. But it has been also considered as substrate for printed electronics by many researchers due to its lower costs and recyclability. Recently, there is a lot of scientific research focusing on the potential use of printing technologies for low
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cost mass production of especially UHF antennas on different substrates. Traditional printing technologies such as screen printing and gravure printing have been proven to be suitable technologies for a wide range of substrates, including paper-based substrates such as corrugated cardboards [1–3]. Research groups investigated also the potential of different paper substrates for printed electronics based on thin films deposited via inkjet technology [4,5]. They proved that certain paper materials are suitable for the inkjet deposition of functional layers, e.g. special inkjet papers equipped with polymer coatings ensuring low roughness and optimized solvent absorption. Denneulin et al. and Puukko et al. demonstrated the optimization of paper substrate surfaces to make it compatible with the deposition of functional layers for printed electronics. The optimization is based on self-made UV-curable inkjet ink formulations to modify different paper types for inkjet printing [12,13]. Denneulin et al. presented the high potential of a UV primer layer for deposited carbon nanotube (CNT) patterns directly on paper-based substrates. We use the same approach, but showing the feasibility of printing silver-based UHF antennas with adapted scattering parameters and industrial relevant inkjet printheads. To the best of our knowledge, the first inkjet-printed UHF antenna on paper substrate was reported in 2007 by Yang et al. [6]. Since then, several scientific papers have been published, e.g. by Amin et al. or Rida et al., focusing on optimized antenna designs regarding the dielectric properties of the used paper substrates and the characteristics of the antenna [7–11]. Next to paper substrates, many publications report about the inkjet printing of antennas on nonabsorbent substrates such as FR4, glass, kapton or PET [14–17]. Virtanen et al. [18] deposited a similar silver ink as used in this contribution on cardboard substrates. They discuss in detail the usage of wood-based substrates and the required adjustments for the inkjet process. The high ink absorptiveness of the substrates is compensated by printing several layers (20 layers for the cardboard substrate) applying a laboratory printhead with 10 pL nominal drop volume [18]. The drawback of this approach is the high silver material consumption and the very long processing time, especially when sintering of each individual silver layer is performed for 60 min. In contrast, we demonstrate a very fast inkjet deposition based on only one silver layer and a UV-curable primer layer. That will limit the consumption of the expensive silver material and the fast deposition facilitates also roll-to-roll processing.
2. Materials and methods The paper-based corrugated cardboard (1.30 B, Dunapeck Spremberg GmbH) employed in this work is a standardized packaging material for the transportation of goods worldwide. The substrate is composed of three different parts: (i) kraft liner top board, (ii) single flute middle part and (iii) test liner bottom board. The flute has a thickness of about 3.2 mm and is made of test liner material. The kraft liner consists of unbleached kraft pulp (sulphate wood pulp) and has a higher quality than the test liner, which is made from recycled paper. For the conductive layer, a commercially available silver ink (SunTronic EMD5603; SunChemical Corporation) was applied. It is a solvent-based nanoparticle ink optimized for jetting by various piezoelectric inkjet printheads. According to the specification of the manufacturer, the silver particle size is in the range of 30–50 nm dispersed in a mixture of ethylene glycol, ethanol and glycerol. The content of silver is about 20 wt%. The ink has a viscosity of 10–13 mPa·s at 25 ◦ C and a surface tension of 27–31 mN/m. For the primer layer, a UV-curable ink (V-Photon Clear NonWet; Tritron GmbH) was employed. It is based on acrylates and allows
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chemical curing by UV radiation in the range of 385–395 nm. Thus, the transfer from a wet layer to a solid layer due to cross-linking and polymerization is photo-chemically induced. The viscosity of the ink is in the range of 16–19 mPa s at 25 ◦ C and the surface tension in the range of 31–33 mN/m at 25 ◦ C (specification of the manufacturer). Both of the inks were deposited with drop-on-demand inkjet systems (DMP 2831, DMP 3000; Dimatix Fujifilm Inc.). The printers operate with piezoelectric printheads and are designed and optimized to print various functional materials. Different printheads were used in the research, depending on the selected ink as well as on the printed pattern dimensions. First investigations have been made for all inks with the Dimatix Material Cartridges (DMC) 11610 consisting of a fluid bag and a laboratory printhead. The printhead is equipped with 16 nozzles arranged in a single line generating drops with a nominal volume of 10 pl. Two different industrial printheads with 128 nozzles arranged in a single line have been used for larger pattern dimensions. The SE3 printhead with a nominal drop volume of 35 pL was employed for printing the silver ink EMD5603 and the SX3 printhead with a nominal drop volume of 8 pL for depositing the UV-curable ink. To functionalize the printed layers, different post-treatment processes were carried out. UV-curing (SubZero 085 ES System, Integration Technology Ltd.) was used to polymerize the UV inks. A standard convection oven (Carbolite Type 200, Carbolite Ltd.) was employed for the drying and sintering of the deposited silver ink at 150 ◦ C for 30 min. CST Microwave Studio was used for the simulation of the RFID antenna parameters. A 3D-model of the antenna design (as shown in Figs. 9 and 11) was created digitally. Next to the antenna geometry following parameters of the silver layer were assumed: Electrical conductivity of 5 × 106 S/m and layer thickness of 2 m. The simulation was done with a standard 50 port impedance to show the antenna functionality independent of any specific RFID chip impedance. Antenna matching for specific RFID chip impedance can be done easily by slight antenna design modification. The antenna characterization was carried out in an anechoic chamber. Relevant antenna parameters such as the scattering parameter S11 were measured with the network analyzer ZVL6 (Rohde & Schwarz). The antenna gain was determined by a spectrum analysis of a communication link between the device under test and a calibrated patch antenna. The antenna was contacted electrically with a conductive adhesive tape (3 M Tape 9703) to a coaxial cable. Scanning electron microscopy (SEM, TM-1000, Hitachi) was performed to analyze the surface of the cardboard substrate. Surface profilometry measurements were done with a Dektak 150 system (Veeco). The absorptiveness of the cardboard substrate was determined with the method according to ISO 535 (Cobb 60 s).
3. Results and discussion 3.1. Corrugated cardboard substrate Only the bottom board (kraft liner) of the corrugated cardboard was used for the deposition of the silver ink. Fig. 1 shows the SEM image of the surface of the bottom board. The difference in size, density and arrangement of the cellulose fibers at the surface of the substrate causes a high arithmetic average roughness of about 9.7 m ± 0.4 m. The water absorptiveness was determined using a Cobb tester according to ISO 535 for 60 s. The obtained water absorptiveness was <33 g/m2 which corresponds approximately to the absorptiveness of standard office papers.
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Fig. 1. SEM image of the kraft liner surface of the cardboard substrate showing the fiber sizes and their arrangement.
Fig. 2. Microscopic image of the edge of a printed and sintered silver pattern; area (a) indicates the printed silver layer and (b) the bare surface of the cardboard substrate.
3.2. Development of primer layer The silver ink was printed on cardboard paper in patterns of 5 mm × 5 mm. By changing the resolution of the printer, different amounts of drops per area can be obtained. The patterns were printed with a resolution of 1270 dpi up to 5080 dpi. Due to the high absorption capacity of the substrate and the high surface roughness, the low viscous silver ink penetrated into the paper for all print resolution. No conductivity was obtained after the thermal posttreatment which was applied to sinter the silver nanoparticles. The high absorption capacity and surface roughness avoid the formation of a conductive network of silver particles. As the microscopic image in Fig. 2 shows, a homogenous silver film has not formed on top of the substrate. The edges of the deposited silver are marked with the dotted line. Area (a) indicates the printed silver layer and (b) the non-coated cardboard surface. Obviously, the edges of the silver pattern are very irregular because of the inhomogeneous ink
Fig. 4. Photographic images of inkjet-printed UV ink Tritron V-Photon NonWet on cardboard with (a) 5080 dpi, (b) 2540 dpi, (c) 1693 dpi and (d) 1270 dpi print resolution, the red rectangle indicates the digital pattern size. (For interpretation of the color information in this figure as well as for the following figures, the reader is referred to the colored web version of the article.)
penetration into and along the paper fibers of the surface. Even increasing the number of stacked layers at 5080 dpi print resolution up to five could not improve the electrical properties. No electrical conductivity could be measured as a result of the high roughness and absorption capacity. In consequence, a primer layer based on the concept of Denneulin [12] was developed. The concept of the primer layer is shown in Fig. 3. An ideal layer formation can be obtained on polymer substrates which have a smooth and non-absorbent surface. A primer layer is required to improve the bad layer formation on cardboard surfaces. The primer layer can ensure a proper layer formation of the silver ink printed on top of it since it reduces the absorptiveness and the roughness of the substrate. A UV-curable ink formulation was selected as primer material to allow the deposition of thick layers as a cardboard roughness of about 9.7 m ± 0.4 m needs to be compensated. Additionally, the UV-curing step can be adjusted to control the ink penetration. The Tritron V-Photon Clear NonWet ink was deposited on the cardboard using the DMP2831 in patterns of 5 mm × 5 mm. Fig. 4 shows the results as a function of print resolution. Following print resolutions were used: Fig. 4 (a) 5080 dpi, (b) 2540 dpi, (c) 1693 dpi and (d) 1270 dpi. In case of 5080 dpi, the layers on top of the cardboard substrate are clearly visible in form of a glossy coating with high thickness. Around the glossy rectangular layers one can see a lot of ink material with low edge sharpness. The reasons are the high absorptiveness of the cardboard and comparable slow printing process with a long time between layer deposition and layer curing. Faster printing and faster curing as usual for industrial print system will allow sharp patterns without excessive material absorption. Decreasing the print resolutions from Fig. 4 (a) to (d) results in thinner layers where most of the material is absorbed by the substrate. To improve the primer layer, a second layer of the ink was printed on top of the already cured UV layer (cured for 9 s using the SubZero 085 ES UV system). Fig. 5 shows the resulting patterns when a second layer with print resolutions of 5080 dpi, 2540 dpi, 1693 dpi and 1270 dpi was printed on the first layer. The first
Fig. 3. Scheme of ink layer formation on different surfaces [based on 12].
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Fig. 5. Photographic images of two layers of inkjet-printed UV ink Tritron V-Photon NonWet on cardboard with different print resolutions for the first and second layer.
column results in irregular pattern formation with sizes much bigger than 5 mm × 5 mm. Additionally, a lot of material is absorbed by the substrate. The third and the fourth columns show partly developed shiny layers, at least in case of 2540 dpi for the first layer. The UV ink was completely absorbed at lower print resolutions. The best result was obtained at a print resolution of 2540 dpi for both layers. It results in a squared layer with reasonable high edge sharpness and comparable low material absorption. The second layer was cured using the same conditions applied for the first layer. The thickness of the developed primer coating consisting of two inkjet-printed UV Tritron V-Photon NonWet layers with 2540 dpi print resolution was measured to about 15 m with an average roughness of 200–300 nm. Fig. 6 shows a comparison of the surface profiles of the bare cardboard kraft liner surface and primer layer surface on top of the kraft liner. The vertical movement of the styles of the Dektak profiler is shown as a function of the measurement length. Obviously, the roughness was remarkably improved. All the peaks of up to 15 m were covered by the printed primer layer which forms a homogeneous and smooth surface.
Fig. 6. Surface profile of kraft liner with and without the printed primer layer.
3.3. Improvement of the primer layer Following, the silver ink was deposited on top of the primercoated kraft liner using inkjet printing. A print resolution of 1270 dpi was used. The pattern was defined as square with the size of 4 mm × 4 mm. Fig. 7 shows the deposited layer on top of the UV ink. It can be seen, that the silver ink merges as agglomerated drop close to the center of the square due to dewetting effects. The primer surface needs to be modified in order to enable the deposition of a homogeneous silver layer on top. To investigate the dewetting, the surface energy of the primer layer was determined as a function of UV treatment of the deposited primer layer. The static sessile drop method was applied to measure the contact angle of different solvents with the primer layer (MobileDrop system, Krüss GmbH). Based on Owen–Wendth–Rabel–Kaelble, the surface energy was calculated. Fig. 8 shows the result of the surface energy as a function of treatment. The primer layer without any treatment has a surface energy of 28.9 mN/m. The polar part has a very low value of about 1.9 mN/m. Additional application of UV irradiation which was used for the curing process of the UV ink anyway, results in an increase of surface
Fig. 7. Agglomerated silver layer on top of primer-coated cardboard.
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Fig. 10. Sheet resistance and layer thickness of EMD5603 printed on top of the primer layer as a function of print resolution.
Fig. 8. Surface energy as a function of surface modification of the UV-cured primer layer (Tritron V-Photon Clear NonWet).
energy. With a UV irradiation of 6 s, the surface energy has been increased to about 35 mN/m with a polar part of approximately 4 mN/m. The obtained surface energy with 6 s additional UV treatment improved the wetting of the silver on top of the primer remarkably. Continuous silver layers were obtained on the UV primer with reasonable conductivity. 3.4. RFID antenna design and manufacturing A special UHF dipole antenna design adapted to the cardboard substrate was developed to validate the applicability for RFID. The designed antenna including the primer layer is shown in Fig. 9. As explained before, the primer layer was printed two times each with a resolution of 2540 dpi. The printing of the UV layers was performed with the DMP3000 equipped with a Fujifilm Dimatix SX3 printhead. The area size of the first UV layer was increased by 1 mm and the size of the second UV layer by 0.5 mm with regard to the silver antenna layer. Each layer was cured by UV irradiation for 6 s. Prior to printing the silver layer, the surface of the primer layer was activated by additional 6 s UV treatment to improve the wetting behavior as explained before. Printing of the silver layer was carried with a Fujifilm SE3 printhead using a print resolution of (i) 847 dpi, (ii) 1016 dpi and (iii) 1270 dpi. The measured sheet resistances of the printed patterns as a function of print resolution and layer thickness are depicted in Fig. 10. The layer thickness was
Fig. 9. Designed layer stack consisting of UV primer layers and the UHF silver antenna pattern.
measured with the Dektak profiler at a printed line with 500 m width. As visible, the sheet resistance decreases approximately linear with the print resolution. Also the layer thickness increased as expected with increasing print resolution. The print resolution will have a significant influence on the performance of the designed UHF antenna. Not only the conductivity influences the antenna parameters, also the geometrical accuracy of the antenna design is of importance. Fig. 11 shows the designed antenna in detail including the most important dimensions. The inset section highlights one of the most important geometrical parameters for the antenna performance: the width of the gap. Here, the width of the gap is marked as (a) if the gap length position is perpendicular to the printing direction and (b) if the gap length is along the printing direction. The digital gap width for (a) is defined as 500 m and the digital gap width for (b) as 650 m. The patterns were printed with the before mentioned print resolutions and the gap widths of the prints were measured with a light microscope. Fig. 12 shows the measured gap widths as a function of printing direction and print resolution. The digital gap widths are indicated with the dotted line. It can be seen that perpendicular to the printing direction the gap widths are almost in the range of the digital dimension. However, the gap width in printing direction is for all print resolutions
Fig. 11. Digital antenna pattern, inset marks the gap width positions (a) perpendicular to printing direction and (b) in printing direction.
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Fig. 12. Gap width of the inkjet-printed UHF antenna on top of the primer layers as a function of print resolution and printing direction.
Fig. 13. Inkjet-printed UHF antenna on cardboard substrate in anechoic chamber for measurement of scattering parameters.
smaller than the digital size. The higher the print resolution, the larger the deviation to the designed digital gap width. Therefore, with the lowest print resolution the highest geometrical accuracy with respect to the designed antenna was obtained. The manufactured UHF antennas were characterized concerning their scattering performance in an anechoic measurement chamber as depicted in Fig. 13. The connection was established with adhesive tape and a coaxial cable to the network analyzer. The scattering parameter S11 was determined during the experiment allowing the deduction of the reflective properties of the UHF antenna. In general, a S11 value in the range of −15 dB and −30 dB normally represents a good adapted antenna qualified for RFID applications. A wave impedance value of 50 has been set for the measurement. The measurement results are plotted in Fig. 14 for each antenna
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Fig. 15. Antenna gain of inkjet-printed antennas as a function of print resolution.
print resolution. Positively, the measured scattering parameter in Fig. 14 (A) show a higher bandwidth compared to the simulated results in Fig. 14 (B). The simulated graph shows a wavy curve next to the peak position due to interactions between the dipol and the director. It can be seen that all antennas printed on the primer layer on the cardboard kraft liner are functional. They have a resonance frequency in the range of 0.988 GHz. Interestingly, the lowest S11 value of about −32 dB was obtained for the lowest print resolution (847 dpi) where the sheet resistance was very high with about 1.78 ± 0.53 /sq. Obviously, the geometrical accuracy has a very big influence since a print resolution of 847 dpi was close to the desired antenna design. With increasing print resolution, the antenna performance decreases as indicated by the S11 parameter. The S11 parameter increases to about −9.5 dB for the antenna printed with 1270 dpi. The performance of the antenna with 1016 dpi is even worse, most probably due the geometrical deviation of the gap width perpendicular to printing direction. This experiment shows that one of the most critical factors for the antenna functionality is the compliance of the antenna geometry. Deviations from the antenna design seem to influence more prominent the antenna performance than the conductivity. Next to the scattering parameter the antenna gain was determined. Fig. 15 shows the antenna gain at 50 as a function of frequency of the inkjet-printed antennas with print resolutions of 847 dpi, 1016 dpi and 1270 dpi. The simulation is shown in the graph as well. There are some deviations comparing the simulation and measured data, especially at lower and higher frequencies. But overall, both the simulation data and the measurement data are in the same range. At a frequency of 0.9885 GHz the antenna
Fig. 14. (A) Measured scattering parameter S11 of inkjet-printed antennas as a function of print resolution and (B) simulated scattering parameter S11 (CST Microwave Studio).
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gain varies between 2 dBi and 2.3 dBi depending on the print resolution of the antenna. The lowest gain was determined for the antenna with a print resolution of 1016 dpi—the same print resolution where the lowest scattering performance was measured as shown in Fig. 14. The obtained antenna gain in the range of 2 dBi is typically comparable to aluminum etched planar RFID transponder dipoles. Thus, we can demonstrate that the inkjet-printed antennas on the cardboard substrates are functional with comparable gain of aluminum-etched dipoles. Therefore, they could potentially operate in RFID applications. The highest antenna gain of about 3.3 dBi was obtained at 1.106 GHz for a print resolution of 1270 dpi. This print resolution results in the highest layer thickness and the lowest electrical resistance compared to 847 dpi and 1016 dpi as shown in Fig. 10. 4. Summary and conclusion The focus of the contribution was set on the development of UHF antennas on corrugated cardboard substrates using inkjet printing. Industrial relevant printheads were chosen for inkjet printing of the different functional materials. In order to facilitate the usage of cardboard substrates for printed electronics, a primer layer was deposited in patterns and optimized towards a homogeneous film formation of the silver layer on top of it. Without this primer layer, we could not obtain a conductive film of silver ink—even with multiple printed layers at high print resolution. A UV-curable, commercially available ink formulation was applied for the primer layer. Deposition of two layers of the UV ink formulation enables the formation of a smooth surface and thus coverage of all the peaks and valleys of the rough cardboard surface. The developed UV ink primer layer was improved concerning its surface energy by additional UV treatment. A special antenna design was developed and printed via inkjet technology on the primer layers. The morphology of the printed silver layers was characterized focusing on the deviations to the digital antenna design. Additionally, the layer thickness was determined as a function of print resolution. The performance of the printed antennas was analyzed in an anechoic chamber based on the scattering parameter S11 and the antenna gain. All of the printed antennas were functional. The best UHF antenna regarding the scattering parameter S11 was obtained with a comparable low print resolution of 847 dpi. Antennas with a print resolution of 1270 dpi resulted in the highest antenna gain of about 3.3 dBi at a frequency of 1.106 GHz. The antenna gain at 0.9885 GHz is in the range of 2 dBi to 2.3 dBi. There is only a minor difference between 1270 dpi and 847 dpi. The low print resolution is advantageous concerning the costs, since a lower print resolution refers to less material consumption. Our results indicate that the geometrical accuracy of the antenna seems to be more important than the conductivity of the antenna. Antennas with a S11 value of −32 dB and antenna gains in the range of 2 dBi at the resonance frequency of 0.988 GHz were manufactured successfully on corrugated cardboards using exclusively inkjet printing. This can be considered as important step towards potential applications for the packaging industry.
Acknowledgements Ralf Zichner was financially supported by the SAB project WeFuFlex. Kalyan Yoti Mitra is acknowledged for his support for preparing the SE3 printhead and his support during printing of the silver layer at the DMP3000. We thank Dr. Siegrun Ulbricht and Mrs. Karin Förster for their help with the Cobb tester. Enrico Sowade was financially supported by the European Commission within the Framework FP7-ICT (grant agreement number 287682, project acronym TDK4PE). References [1] T. Muck, M. Staresinic, D.G. Svetec, M. Pivar, U. Kavcic, M. Staresinic, Printed HF and UHF RFID antenna directly on cardboard and recycled paper, LOPE-C (2012) 398–401. ¨ [2] S. Merilampi, L. Ukkonen, L. Sydanheimo, P. Ruuskanen, M. Kivikoski, Analysis of silver ink bow-Tie RFID tag antennas printed on paper substrates, Int. J. Antennas Propag. (2007) 1–9. [3] M. Rebros, E. Hrehorova, B. Bazuin, Rotogravure printed UHF RFID antennae directly on packaging materials, in: TAGA 60th Annual Technical Conference (2008) 292–304. [4] T. Öhlund, J. Örtegren, S. Forsberg, H.-E. Nilsson, Paper surfaces for metal nanoparticle inkjet printing, Appl. Surf. Sci. 259 (2012) 731–739. [5] L. Xie, M. Mäntysalo, A.L. Cabezas, Y. Feng, F. Jonsson, L.-R. Zheng, Electrical performance and reliability evaluation of inkjet-printed Ag interconnections on paper substrates, Mater. Lett. 88 (2012) 68–72. [6] L. Yang, A. Rida, R. Vyas, M.M. Tentzeris, RFID Tag and RF structures on a paper substrate using inkjet-printing technology, IEEE Trans. Microwave Theory Tech. 55 (2007) 2894–2901. [7] Y. Amin, S. Prokkola, B. Shao, J. Hållstedt, H. Tenhunen, L. Zheng, Inkjet Printed Paper Based Quadrate Bowtie Antennas For UHF RFID Tags, in: 11th International Conference in Advanced Communication Technology (2009) 109–112. [8] M.M. Tentzeris, Inkjet-printed paper-based RFID and nanotechnology-based ultrasensitive sensors: The ‘Green’ Ultimate Solution for an ever improving Life Quality and Safety!?, in: IEEE Conference on Radio and Wireless Symposium (2010) 120–123. [9] A. Rida, L. Yang, R. Vyas, M.M. Tentzeris, Conductive inkjet-printed antennas on flexible low-cost paper-based substrates for RFID and WSN applications, IEEE Antennas Propag. Mag. 51 (2009) 13–23. [10] G. Shaker, S. Safavi-naeini, N. Sangary, A.P. Setup, Inkjet printing of Ultrawideband (UWB) antennas on paper-based substrates, Antennas Wireless Propag. Lett. 10 (2011) 111–114. [11] M. Marroncelli, D. Trinchero, Concealable, low-cost paper-printed antennas for WISP-based RFIDs, in: IEEE International Conference on RFID (2011) 6–10. [12] A. Denneulin, J. Bras, A. Blayo, C. Neuman, Substrate pre-treatment of flexible material for printed electronics with carbon nanotube based ink, Appl. Surf. Sci. 257 (2011) 3645–3651. [13] P. Puukko, A. Ilmonen, T. Lamminmäki, S. Sundqvist, V.T.T. Technical, UV-inkjet Ink Penetration and Its Effect on Print Quality Formation and Drying, NIP 25: International Conference on Digital Printing Technologies and Digital Fabrication (2009) 566–569. [14] Y. Al-Naiemy, T.A. Elwi, H.R. Khaleel, A. Hussain Al-Rizzo, Systematic approach for the design, fabrication, and testing of microstrip antennas using inkjet printing technology, ISRN Commun. Network. (2012) 132465. [15] V.K. Palukuru, A. Pekonen, V. Pynttäri, R. Mäkinen, J. Hagberg, Jantunen Heli, An inkjet-printed inverted-F antenna for 2.4-GHz wrist applications, Microwave Opt. Technol. Lett. 51 (2009) 2936–2938. [16] J. Virtanen, J. Virkki, A.Z. Elsherbeni, L. Sydänheimo, L. Ukkonen, A selective ink deposition method for the cost-performance optimization of inkjet-printed UHF RFID tag antennas, Int. J. Antennas Propag. (2012) 801014. [17] M. Mäntysalo, P. Mansikkamäki, An inkjet-deposeted antenna for 2.4 GHz applications, Int. J. Electron. Commun. 63 (2009) 31–35. [18] J. Virtanen, J. Virkki, L. Ukkonen, L. Sydänheimo, Inkjet-printed UHF RFID tags on renewable materials, Adv. Internet Things 2 (2012) 79–85.