Ink-jet Printed CPW Inductors in Flexible Technology

Aleksandar Menicanin*, Ljiljana Zivanov, Mirjana Damnjanovic, Andrea Maric and Natasa Samardzic *Institute for Multidisciplinary Research, University of Belgrade, Belgrade, Serbia

Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia email: aleksandar.menicanin@imsi.rs

Abstract— In this paper we present an ink-jet printed coplanar waveguide (CPW) inductors in flexible technology. We show design, fabrication, characterization and measurement technique of such devices. This is solution for ultra low-cost mass production. CPW inductors were developed as a technology for much simpler and faster fabrication on/in plastic foil. Structures are printed on Kapton substrate with polyamide silver nanoparticle ink.

Keywords— Flexible technology, inductors, ink-jet printing, printed electronics

I. INTRODUCTION Printed flexible technology is one of the best methods for

making very precision circuits on flexible printed circuits boards (flexible PCBs). This technology is very usable for fabricating printed electronics, solar cells, mobile phones, Radio Frequency Identification (RFID), Wireless Local Area Networks (WLAN) antennas, smart cards, etc. It represents low-cost green technology, and most of the applications are printed in one layer.

As a demand for low cost, flexible and efficient electronics increases, the materials and integration techniques become more and more critical and face more challenges, especially with the ever growing interest for "cognitive intelligence" and wireless applications, such as RFID, WiFi, WiMax. For an optimal RF performance, substrate material characterization for electronics such as antennas, filters and transmission lines becomes a must, especially for low-cost materials. A fast process, like ink-jet printing, can be used efficiently to print electronics on/in plastic or paper substrates [1, 2].

Printed electronics is gaining wide momentum owing to the many benefits it offers over conventional electronics, such as low fabrication cost, long switching times, and simple fabrication. This technology has found use in a plethora of applications ranging from displays and lighting to RFID, sensors, and batteries. The global printed electronics market is expected to reach $24.25 billion by 2015. Increased miniaturization, technological changes, and portability needs of electronic products in different sectors such as telecommunications, packaging, automotive, and medicine are driving the demand for flexible electronic products in the market [3]. There are significant growth opportunities for printed electronics in end-user markets such as consumer

electronics, military, power generation, healthcare, and logistics.

Low-resistance printed conductors are crucial for the development of ultra-low cost electronic systems and components such as high-Q inductors, capacitors, tuned circuits, and interconnects. Because of that, we have designed coplanar waveguide (CPW) inductors without back ground metallization. The goal of this research is to transfer knowledge previously gained on fabrication in non-flexible technologies. We developed meander-type inductors in low temperature co-fired ceramic (LTCC) and different type inductors in ceramic co-processing technology [4, 5], and passive components in flexible technology [6].

The electrical resistance of printed metal lines is dependent on a number of variables: type of nanoparticle, printed thickness, and sintering temperature. Printed thickness depends on: wetting characteristics of the substrate surface, dot size, substrate temperature, and viscosity [7].

In this paper, we present CPW inductors in flexible technology. In Section II is presented design and ink-jet fabrication process of CPW inductors on plastic Kapton foil. In section III are shown measurement results and characterization of presented structures. The reached conclusions of the obtained results follow finally in the Section IV.

II. DESIGN AND INK-JET FABRICATION PROCESS OF CPW INDUCTORS

A. Design of CPW Inductors Design, modeling and simulation of CPW inductors are

obtained using electromagnetic simulator, built in software package Microwave office (MWO, AWR Corp).

Designed CPW inductors are two-port meanders with 1, 1.5 and 3 turns (n number of turns). Line paths in presented structures have width w = 200 µm. Around CPW inductors structure is ring-shape ground plane, which is replacement for background metallization. In that manner, structures are manufactured in low-cost one-layer fabrication process.

Designed CPW inductor on flexible substrate is presented in Fig. 1. All geometrical parameters of interest are clearly marked: the width of the conductive line w, the spacing between adjacent segments s, outer dimensions of the inductor

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dx and dy, and the total thickness of the substrate CPW inductors d. In this case, d is thickness of Kapton substrate.

3D electromagnetic (EM) model of CPW inductor, modeled in MWO, contains an additional top air box that is required for defining the boundaries of the EM enclosure. The top boundary was modeled as “approximately open.”

1 2w

d

s

dx

dy

air box

Fig 1. Geometric structure of CPW inductor meander type with 1.5 turn:

top view and 3D model (built in MWO).

Geometrical parameters of designed CPW inductors are presented in the Table I. Ring ground is rectangular path with 400 µm line width around meander structures. Depending on the number of turns n, we obtain the projected value of inductance, which is directly related to the final dimensions of the CPW inductors.

TABLE I. GEOMETRICAL PARAMETERS OF FABRICATED CPW INDUCTORS

CPW1x200 CPW1.5x200 CPW3x200

n (turn) 1 1.5 3

dx (mm) 1.8 2.2 3.4

dy (mm) 2 2 2

d (µm) 75 75 75

s (µm) 200 200 200

w (µm) 200 200 200

Parameters of material used for inductors’ fabrication influence on their performance. Influence of the most important parameters of the substrate such as dielectric loss (tan δ) and relative permittivity (εr) are especially significant. Used plastic foil Kapton has dielectric loss tan δ = 0.0019 and relative permittivity εr = 3.2 [8].

B. Fabrication Process: Ink-Jet Printing of CPW Inductors Printed electronics uses different printing techniques,

namely ink-jet, screen, flexography, gravure, offset lithography, nano-imprint lithography, and some proprietary printing techniques developed by market players. Printed electronics technology makes use of conductive and dielectric inks. Plastic, glass, and paper are the commonly used substrates for developing printed components [3].

CPW inductors in flexible technology are printed on plastic/organic Kapton substrate (75 µm thick) with silver nanopatricle ink, using ink-jet printer Dimatix Materials Printer DMP-3000 [9], as shown in Fig. 2. To ensure the ink droplets sufficiently overlap, 25 µm drop spacing was selected. Drop spacing determines the thickness of printed layers. In Fig. 3 are presented ink-jet printed CPW inductors on flexible substrate with real dimensions, and enlarged section.

The ink used here contains silver nanoparticles capped with a polymer coating that keeps the particles in a colloidal suspension. Inks with other metallic nanoparticles such as gold are also available. Once the silver ink has been printed, it can be sintered at a low temperature (100°C-300°C) to form electrically conducting lines, enabling low temperature substrates such as paper [1] and polyimide to be used.

After printing process, structure goes in furnace for sintering process at 200ºC for 40 minutes. The conductivity of the conductive ink varies from 0.4-2.5x107 Siemens/m depending on the curing temperature [10]. Expected conductivity of printed silver layers with presented printing and sintering process is below 1x107 Siemens/m.

After ink-jet printing, a low temperature sintering step guaranteed a continuous metal conductor, providing a good percolation channel for the conduction electrons to flow.

Fig 2. Dimatix Materials Printer DMP 3000 [8].

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Fig 3. Ink-jet printed CPW Inductors on flexible substrate (in cm).

III. MEASUREMENT RESULTS AND CHARACTERIZATION OF CPW INDUCTORS

Realized structures are measured with Agilent N5230A PNA-L vector network analyzer (VNA), Süss MicroTec RF probe station PM5, and coplanar ground–signal–ground (GSG) Cascade Microtech probes (|Z| probes). The measurement system was calibrated using SOLT (Short-Open-Load-Thru) calibration technique using Cascade Microtech’s calibration substrate 101-190. Measurement results are obtained in frequency range from 1 GHz to 35 GHz. Measurement set-up and GSG probe pads in one moment of measurement are shown in Fig. 4.

Fig 4. Measurement setup and structure in one moment of measurement.

After calibration of the measurement system, measurement of scattering parameters (S-parameters) of fabricated inductors has been performed.

From S-parameters we extracted basic electrical parameters of inductors, such as inductance, and Q-factor. From S-parameters, through Y and Z matrix we obtained presented results for inductance and Q-factor.

Effective inductance L and effective quality factor in two-port measurement can be calculated by using input impedance through Y matrix parameters as follows

11 22 12 21

4Im

2Y Y Y Y

Lfπ

+ − − = , (1)

( )( )

11 22 12 21

11 22 12 21

ImRe

Y Y Y YQ

Y Y Y Y+ − −

= −+ − −

. (2)

For symmetric inductors, Y22 and Y21 are approximately equal to Y11 and Y12, respectively, so the following approximated equation s can also be utilized as shown in [11]

11 12

2Im

2Y Y

Lfπ

− ≅ , (3)

( )( )

11 12

11 12

ImRe

Y YQ

Y Y−

≅ −−

. (4)

Calculated values of effective inductance (at 1 GHz and quality factor maximum frequency fQmax) and quality factor Q extracted from measured results of all CPW inductors are presented in Table II.

TABLE II. EXTRACTED PARAMETERS FROM MEASUREMENT RESULTS

CPW1x200 CPW1.5x200 CPW3x200

L at 1 GHz (nH) 1.959 2.586 4.457

L at fQmax (nH) 1.78 2.53 4.52

Qmax 3.5 3.167 2.231

fQmax (GHz) 6.865 6.27 3.656

Measurement results of CPW inductors in flexible technology, such as inductance and Q-factor, are presented in Fig. 5 and Fig. 6. From these measurement results we can see similar behavior with inductors in different technology.

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By increasing the number of turns (n) the inductors will have greater inductance (see Fig. 5), which cause the reduction of the self resonant frequency. Because of that, parasitic capacitance became larger. Q-factor of fabricated CPW inductors is in the range of 2-3.5 (see Fig. 6), which is relatively low value compared to different technology, like LTCC. With increasing the number of turns n, the Q-factor reduces, and its maximum moves to lower frequencies.

The maximum value of Q-factor affects conductivity of printed silver-ink. Maximum value of Q-factor could be increased in two different ways. One of these is increasing contents of silver particles in polymer ink enhancing material conductivity, or usage of lower resistive ink. Another way is multiple printing of conductive lines with intent to obtain thicker conductive lines.

-8

-6

-4

-2

0

2

4

6

8

0 5 10 15 20 25 30 35

L (nH

)

Frequency (GHz)

CPW 1x200CPW 1.5x200CPW 3x200

Fig 5. Inductance of meander type CPW inductors.

0

1

2

3

4

0 5 10 15 20

Q-fa

ctor

Frequency (GHz)

CPW 1x200CPW 1.5x200CPW 3x200

Fig 6. Q-factor of meander type CPW inductors.

IV. CONCLUSION Ink-jet printing shows a very promising future for ultra-

low-cost fabrication process of passive microwave circuitry and interconnects.

This paper presents the CPW inductors made in flexible PCBs technology. The results of inductor parameters are given, such as inductance and Q-factor. These devices have high operating and self resonant frequencies, up to 17 GHz. Q-factor has somewhat smaller value. If we want to achieve higher Q-factor, reduction of serial resistance of CPW inductors and/or increase the thickness of the layer or increase the concentration of silver containing in polymer silver-ink needs to be performed.

ACKNOWLEDGMENT This work was supported in part by the Ministry of

Education and Science, Republic of Serbia, under project TR-32016 and EC FP7 REGPOT project APOSTILLE, grant no. 256615. The authors would like to acknowledge Applied Wave Research Ltd., UK, for providing its powerful software tools.

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[4] G. Radosavljević, A. Marić, W. Smetana, Lj. Živanov: “Benefits of the LTCC Substrate Configuration with an Air-Gap for Realization of RF Inductor with High Q-Factor and SRF”, Int. Jour. of Applied Ceramic Technology, December 2011.

[5] Stojanović G., Damnjanović M., Desnica V., Živanov Lj., Raghavendra R., Bellew P., Mcloughlin N.: “High Performance Zig-Zag and Meander Inductors Embedded in Ferrite Material”, Journal of Magnetism and Magnetic Materials, Vol. 297, No 2, pp. 76- 83, 2006.

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[7] O. Azucena, J. Kubby, D. Scarbrough, C. Goldsmith: “Inkjet printing of passive microwave circuitry”, IEEE MTT-S International Microwave Symposium Digest, pp: 1075 – 1078, Atlanta, GA, 15-20 June 2008.

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