Inkjet-printing of Hybrid Ag/Conductive polymer towards strechable microwave devices

Sebastien Pacchini*1, Mathieu Cometto*2, Jian Jie Chok*3, Gaetan Dufour+4, Nicolas Tiercelin+5, Philippe Pernod+6,

Tay Beng Kang*7, Philippe Coquet*+8 * CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore

637553 * School of Electrical and Electronics Engineering, Nanyang Technological University, Block S1, 50 Nanyang Avenue, Singapore

639798 1spacchini@ntu.edu.sg, 2MATHIEU001@e.ntu.edu.sg, 3chok0009@e.ntu.edu.sg, 8philippe.coquet@iemn.univ-lille1.fr

+Joint International Laboratory LICS/LEMAC, IEMN UMR CNRS 8520, Centrale Lille, PRES : "Univ. Lille"

59651 Villeneuve d’Ascq, France 4Gaetan.Dufour@iemn.univ-lille1.fr, 5nicolas.tiercelin@iemn.univ-lille1.fr, 6philippe.pernod@iemn.univ-lille1.fr

Abstract— Two microwave structures (microstrip transmission line (TL) up to 10 GHz and a patch antenna @ 13 GHz have been fabricated on a flexible Polydimethylsiloxane (PDMS Sylgard 184) substrate by inkjet-printing. To overcome the mechanical robustness issue of the metal parts under stretching, inclusions of composite films of multiwall carbon nanotube (MWNT) with poly (4-styrenesulfate) (PEDOT:PSS) are formed in the TL lines. The performances of these structures are compared with a similar silver printed microstrip TL and a silver printed patch antenna without the composite inclusion. The experimental results are in good agreement with the simulations. The increase of the insertion losses of the hybrid structures is kept below 1dB/cm for inclusions of 500 µm. The results confirm the potential of MWNT/PEDOT:PSS for stretchable microwave devices.

Keywords— microstrip transmission lines, flexible antennas carbon nanotubes, PEDOT:PSS, PDMS, inkjet-printing, elastomer

I. INTRODUCTION Printed electronics is a rapid prototyping technology

emerging within the R&D manufacturing. It is set to revolutionize the fabrication of the electronic devices on flexible substrate materials such as plastic, paper, and textile using electrically functional inks. The next-generation of ubiquitous RF electronics [1] such as antennas [2]-[3], impedance matching [4]-[5], EMI protection [5], smart tags coupled with sensors [7]-[8], filters and resonators [9]-[10] and RF-TFT [11] requires materials that are flexible enough to conform to any desired shape as well very low-cost processing techniques.

With the emergence of nano-particles and printable nano-objects, new classes of tunable materials have entered the R&D manufacturing landscape. The use of materials based on carbon nanotubes (CNT) appears to be very promising for nanotechnology applications presented earlier [9-15]. CNT has shown exceptional stiffness, and remarkable thermal, electrical and mechanical properties. These advantages make them an ideal candidate for the development of multifunctional material for functional devices. Elastomers like Polydimethylsiloxane (PDMS) have become increasingly popular for stretchable and reconfigurable RF applications because of their extreme flexibility (EYoung=2 MPa for PDMS) [3]. Besides, PDMS presents many attractive properties: it is chemically inert, watertight, transparent and biocompatible. It can shaped to any form and thickness, and its wettability can be reversibly adjusted via oxygen plasma treatment.

In this communication, we describe a process that uses the inkjet-printing technique to print conductive films based on Ag nanoparticles and MWNT/PEDOT:PSS, in order to achieve the fast prototyping of microwave devices on flexible PDMS substrate. In part II, the design of a microstrip transmission line (TL) and of an antenna is presented. The fabrication process of elastomeric supported microwave devices is detailed in part III. Finally, in part IV, measurement results and comparisons with simulations are presented for transmission lines and antennas working respectively up to 10 GHz and 13 GHz .

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II. RF DESIGN Microstrip devices on PDMS (a 50 Ω TL and a patch

antenna) have been designed using Ansys HFSS. PDMS (Sylgard 184, Dow Corning) presents dielectric properties of εr = 2.68, tan δ = 0.04 at millimeter-waves [16-21]. Schematics and dimensions of the devices are given Fig.1. The microstrip patch antenna has been designed to operate at 13 GHz while printed on a 360-µm-thick PDMS substrate.

Two kinds of devices were simulated: i) full Ag reference devices (for TLs and patch antenna) and ii) devices with MWNT/PEDOT:PSS inclusions. The inclusion length is measured as the gap in the line, but the MWNT/PEDOT:PSS is printed overlapping the lines by 200µm in length and 100µm in width, for better contact purposes, as shown on Fig. 1. The lengths depicted in Fig. 1 are the optimum calculated values, subject to slight variations due to fabrication processes. As for the lines, they are simple microstrip lines of 500µm width on an 180µm thick PDMS substrate.

380 PDMS

Line%In%&%quarterwave matching

Fig. 1. Top: Top view of patch antenna with MWNT/PEDOT:PSS inclusion in the matching line (dark colour). Bottom: Side view of the PDMS substrate with the antenna printed on top. All lengths are scaled in micrometers. The Line IN is designed to be 50Ω at 13GHz, hence the length is not stated.

The DC conductivity of the printed silver ink has been evaluated to 2.4 x106 S/m leading to a skin depth of ~3.3 µm at the working frequency. For practical reasons, a silver ink thickness of 2 µm has been used. The ground planes are made of 40 µm thick copper foils. The DC conductivity of the MWNT/PEDOT:PSS has been evaluated to 15000 S/m.

III. INKJET PRINTING FABRICATION The main technological fabrication steps are described in

Fig.2. First, 180µm (for TL) and 360µm thick (for antenna) films of PDMS were prepared by spin coating on a 3” silicon wafer coated with a C4F8 anti-adhesive layer. The PDMS layers were cured at low temperature at 45°C for 4 hours to avoid excessive thermal mismatch with the substrate. Prior to inkjet printing, the PDMS has then to be submitted to an oxygen plasma: the wettability of surfaces is a critical parameter that has to be carefully controlled to print the conductive materials. PDMS is a hydrophobic material that exhibits a static contact angle of 115˚. Under 10 minutes of oxygen plasma, the contact angle measured on the PDMS surface decreases down to the range of 50-40˚ which is suitable to print Ag ink and MWNT/PEDOT:PSS ink.

Microwave devices are printed using a Dimatix Materials Printer (Model DMP-2800, FUJIFILM Dimatix, Inc. Santa Clara, CA).

a) b) Antenna

Microstrip

Fig. 2. Schematic view of the fabrication process of a microstrip line and an antenna by inkjet-printing, a) PDMS preparation, b) printing process of passive elements (microstrip TL and antenna) on PDMS.

To compare and evaluate the performance of hybrid microwave structures, identical full (i.e. without inclusion) Ag microstrip TLs and Ag patch antennas on PDMS have also been printed.

A. Printing of microstrip line & antenna Using the Dimatix Printer (www.dimatix.com), the

microwave devices (microstip TL and antenna, show Fig. 2.b) are directly printed on demand onto the PDMS surface. The SunTronicTM Jettable silver-based ink manufactured by SunChemicals (Product DGP 40LT-15C, Anexus) is an ink that is widely used in the industry of flexible electronics, owing to its excellent jettability through various piezoelectric inkjet printheads. It is compatible with various substrates such as paper, kapton, glass and PDMS. The silver (Ag) nano-particles are provided in ethylene glycol/ethanol whereas the solution has been characterized as having 30 wt.% silver, with an average particle size of 20-25nm, a dispersion viscosity of 14 cPs at 25°C and a surface tension of 35-38 mN/m at 25°C. A 2µm thick silver film is printed on the surface of PDMS with gaps of 496µm and 957µm (Fig.3.a) for the realization of the inclusions. The samples are then subsequently cured at 120°C for 1 hour to remove the remaining solvents, to provide the percolation channel of Ag nanoparticles and to improve the electrical conductivity of the metallization.

B. Inkjet Printing of MWNT/PEDOT:PSS on PDMS The HC Poly-InkTM jettable based ink is an aqueous,

conductive and transparent printable ink manufactured by Poly-Ink. The carbon nanotubes (MWNT) and conductive polymer (PEDOT:PSS) allow the printing of flexible films

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with high transparency and electrical conductivity. The MWNT/ PEDOT:PSS can be printed on many substrates, and polymer films. Depending on the number of printed layers, the conductivity can reach 15000 S/m. It is used here for the realization of the inclusions in the microstrip lines and in the feeding line of the antennas, as shown on Fig.3.b. An overlap of 200µm is realized to enhance the electrical and mechanical contact between Ag and MWNT/PEDOT:PSS sections.

Fig. 3. Microscop images of Microstrip TL printed on PDMS a) Ag microsstrip TL without MWNT/PEDOT:PSS film, b) 5 layers of MWNT/PEDOT:PSS film printed into gap (957µm)

At the end of the fabrication, the inkjet printed devices are cut out of the wafer with a scalpel blade and transferred onto 40 µm thick copper foils ground planes. Devices are shown in Fig. 4.

IV. MEASUREMENTS RESULTS SMA connectors are used to feed the TL and the antenna,

as shown on Fig. 4. S parameters measurements are performed after a TRL calibration, using a vector network analyzer (Rohde & Schwarz ZV24). The losses of TL and the reflection losses (S11) of antennas are shown on Fig.5.

Fig. 4. Microwave characterizations of Ag based and hybrid Ag/MWNT/PEDOT:PSS based devices on PDMS substrate, (a) Ag transmission line, b) Ag transmission line with inclusion of MWNT/PEDOT:PSS, (c) Ag microstrip patch antenna prototype, d) Ag microstrip patch antenna prototype with inclusion of MWNT/PEDOT:PSS

The insertion losses are compared between two structures. The first one is a plain silver microstrip TL with a length of 2cm (Fig. 4.a). The second one is a silver microstrip TL with the same length but with inclusions of MWNT/PEDOT:PSS, (Fig. 4.b). Two lengths have been considered for the inclusions, namely 496µm and 957µm. From the S12-parameter, the first structure demonstrates a loss of -1.58 dB/cm and -4.84 dB/cm, at 5 GHz and 10GHz respectively.

On the second structure, the measurements (Fig.4.b) show the increase of the transmission losses due to the increase of the length of the MWNT/PEDOT:PSS inclusion, the conductivity of the composite film being low compared to silver. For the 496µm-long inclusion, the losses are -2.11 dB/cm and -7.54 dB/cm, for 5 GHz and 10GHz respectively. For the 957µm-long inclusion, the losses are -3.39 dB/cm and -8.12 dB/cm, for 5 GHz and 10GHz respectively. For comparison the losses for the silver lines are -1.58 dB/cm at 5 GHz and -4.84 dB/cm at 10 GHz.

Fig. 5. Computed and measured insertion loss |S12| (dB/cm) of inkjet-printed TL without or with MWNT/PEDOT:PSS inclusions (496µm or 957µm)

Fig. 6. Computed and measured return loss |S11| dB of a microstrip patch antenna without or with 1mm of MWNT/PEDOT:PSS inclusion

The difference between simulations and measurements is attributed to ink surface roughness and fabrication tolerance, which might be further optimized and effects of the connectors not taken into account in the simulations. Besides, according to HFSS simulations, the theoretical transmission loss can be optimized up to 0.5 dB/cm at 4.5 GHz by raising the thickness of the Ag ink to 5µm and micromachining the PDMS substrate in a membrane over air-cavity configuration such as the one in

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[15]. The insertion losses are indeed mainly due to the dielectric losses in the bulk PDMS substrate.

For the microstrip patch antenna, the |S11| has been measured for two antennas prototypes (Ag and hybrid Ag/ MWNT/PEDOT inclusions), as shown in Fig.4.c and Fig.4.d. For the first one, a frequency shift of 200MHz is observed on Fig.6. due to the fabrication tolerances. For the second antenna that was printed with a MWNT/PEDOT:PSS inclusion of 1mm, the agreement between measurements and simulations is also fairly good. The relative shift between the predicted (13 GHz) and measured (13.2 GHz) resonant frequencies is less than 1%.

V. CONCLUSION We have demonstrated here the design, fabrication and

measurement of very low-cost and very fast prototyped microwave inkjet-printed devices. The devices have been successfully printed on ultra-soft PDMS substrates with an optimized surface treatment of PDMS, as well as careful ink formulation.

S-parameters measurements are in good agreement with the modeling, validating this fabrication technique. To improve the transmission and radiation characteristics of the devices, three routes can be investigated: improve the surface roughness of the ink, increase the thickness of the Ag metallization in order to decrease the skin effect, and use PDMS in a membrane configuration over an air cavity to decrease the losses due to the PDMS substrate. Additionally, different types of ink will be studied and the resulting performances compared to the ones of Ag-ink based devices.

Potential applications range from mechanically reconfigurable RF devices to millimeter-wave Wireless Body Area Networks (WBANs) enabled by tiny, ultra-conformable, low-cost and easily fabricated devices.

ACKNOLEDGMENTS Moreover, we thank EEE School from NTU University for

their contributions, the French nanofabrication network RENATECH, the CPER-CIA (Contrat Plan Etat-Région Campus Intelligence Ambiante) and the French “Equipment of Excellence” ANR-11-EQPX-0025 Equipex LEAF.

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