Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer Academic research paper on "Materials engineering"

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Engineering

Procedía Engineering 47 (2012) 1173 - 1176 =

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Proc. Eurosensors XXVI, September 9-12, 2012, Krakow, Poland

Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer

F. Molina-Lopez *,D. Briand, N. F.de Rooij

Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Sensors, Actuators and Microsystems Laboratory (SAMLAB), Rue Jaquet-Droz 1, CH-2002 Neuchâtel, Switzerland

Abstract

A new fabrication strategy is proposed to reduce the size of sensors based on inkjet-printed interdigitated electrodes (IDE). The process is compatible with fabrication at low temperature and can be applied to flexible organic foils. The new fabrication concept consists of the deposition of a thin dielectric layer of parylene-C onto the first electrodes comb prior to the printing of the second one. In this way, the combs are not longer on the same plane, preventing undesired short-circuits and solving the typical problems of definition and yield associated to inkjet printing of IDE devices with small pitch. The proposed strategy results especially relevant for capacitive-based devices, where short-circuits render the device non-functional. Moreover, it permits enhancing their capacitance per surface area. The validation of the exposed process for sensors application has been carried out by functionalizing the IDE structure with a humidity sensing layer, and characterizing the resulting sensor against changes in relative humidity (R.H.).

© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

Inkjet printing; flexible substrate; parylene; gas sensor

1. Introduction

New concepts like intelligent packaging or the "internet of things" are recently gaining a lot of interest. The idea of having a large number of intelligent distributed objects able to sense their environment and communicate between them suggests unlimited new useful applications. One of the principal challenges of these concepts is to bring down the price of every single item to feasible values. In this scenario, plastic and printing electronics find a great opportunity to be studied and developed. The advantageous cost and good mechanical / electrical performances of polymeric substrates make them ideal candidates for the fabrication of inexpensive intelligent systems. On their side, printing technologies in general and inkjet printing in particular are cost efficient, and easy to process. Inkjet printing is also maskless, contactless, suitable for prototyping, and additive. Nonetheless, its behaviour is ruled by the properties of the jetted fluid and its interaction with the substrate [1], making the process relatively variable and hard to control. That fact results in relative low printing definition, being especially critical for IDE transducers (widely used for sensors fabrication). Despite the novelty of the field, we can already find some inkjet-printed IDE in the literature, presenting rather large inter-electrodes gaps ranging from 34to 200 ^m [2-4].

In this work, we propose a new fabrication strategy to increase the value of capacitance per surface area of IDE devices inkjet printed on foil in a reliable way, i.e., without decreasing the fabrication yield. The strategy consists of the deposition of a thin dielectric layer of parylene-C onto the first printed electrodes comb prior to the printing of the

* Corresponding author. Tel.: ++41-32-720-54-34; fax: ++41-32-720-57-11. E-mail address: francisco.molinalopez@epfl.ch.

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

doi:10.1016/j.proeng.2012.09.360

second comb, in a way that they cannot be in contact (Fig. 1). With this new configuration, the printing process results less demanding and the IDE can be more closely packed (since overlapping patterns no longer form short-circuits) enhancing the capacitance of the device. The validity of the elicited strategy for development of higher sensitivity gas sensors was assessed by coating the IDE devices with a humidity sensing layer, and measuring their response against different levels of R.H.

2. Experimental

2.1. Fabrication

Silver ink Suntronic JET EMD5603 has been inkjet-printed with a printer Dimatix DMP 2800 on a polyethylene terephthalate (PET) film (Melinex® ST506 from Dupont) to define the first comb. Previous to the printing step, a brief microwave (2.456 GHz) oxygen plasma treatment was performed on the substrate at 400 W during 15 seconds to improve ink wettability. Following, the lines were sintered for two hours in an oven at 150 °C to totally remove the organic residual present in the ink. A parylene-C layer of 2 ^m thick was then deposited by chemical vapour deposition onto the first electrodes comb. After parylene-C deposition, a plasma treatment similar to the described above was carried out to increase the adhesion between parylene-C and the subsequent cellulose acetate butyrate (CAB) sensing layer. The second comb was printed on the parylene layer and sintered as before. Finally, the sensing layer was developed by dissolving CAB polymer in hexyl acetate in such a way that the solution viscosity and surface tension were suitable for inkjet printing or spray coating.

2.2. Characterization

The variation in capacitance versus R.H. for the fabricated devices were measured at room temperature using a customized gas mixing system with automatic control of wet and dry gas flow inside a small gas chamber. The total gas flow was 500 ml/min. Two LCR-meters were used to measure the corresponding capacitance values at a frequency of 100 kHz at every 5 seconds.

Sensing layer

Inkjet printing ■ nozzle I

Conductive A ink drop ^

Passivation layer

Deposition sensing layer

Substrate

Sensing layer

Printed bottom .electrode

(a) (b)

Fig. 1. Sketch showing the exploded view of the fabricated devices; (b) cross sectional view of such devices

3. Results

+ Substrate - +

Combed electrodes 40.8 ± 0.6 ^m wide and with different inter-electrode gaps have been printed on PET. After parylene-C deposition, a second batch of combs with the same layout and width of 63.2 ± 0.7 ^m was aligned in between the electrodes of the first combs and printed on the parylene-C layer. Table 1 collects the dimensions of every fabricated device and the resulting capacitance value. The real final gap size cannot be properly controlled due to the poor placement accuracy offered by the DMP-2800 printer, which only warranties a pattern alignment reproducibility of ± 25 ^m. Figure 2(a) shows the different IDE structures described in Table 1. Although overlapping occurs between combs due to small pitch or printing defects (see Fig. 2(b)), the two electrical branches are not in contact thanks to the parylene-C interlayer, keeping the devices fully functional. Arrays of printed IDE capacitors on flexible PET are depicted in Fig. 2(c).

Fig. 2(d) shows the theoretical enhancement, of almost a factor ten, of capacitance per surface area when decreasing the inter-electrodes gap of the IDE structures presented above. The evaluation of the capacitance per surface area has been done analytically by adapting the model proposed in [5] for electrodes with our two different widths: 40.8 ± 0.6

^m and 62.3 ± 0.7 ^m. For the sake of simplicity, the layer of parylene-C has been ignored in the capacitance calculation. This approximation is justified considering the similar dielectric constants of substrate and parylene-C as well as the thinness of the parylene-C layer. Due to the poor alignment offered by the printer, it is challenging at this point to fit the experimental values with the model, especially for small gaps. Nevertheless, the potential of the method still remains valid for the use of more complex inkjet printers or in any case, to fabricate functional capacitors with high capacitance per surface area as demonstrated in Table 1.

Table 1. . Electrodes pitch of the first and the second printed combs for every kind of fabricated device

Device Gap 1st comb (pm) Gap final device (pm) C(pF)

A 40.1 ± 1.2 Overlapping 4.25 ±0.15

B 81.6 ± 2.5 9 ±25 1.58 ±0.09

C 122.2 ±2.4 30 ±25 0.6±0.1

D 156.7 ±2.4 47 ±25 0.31 ±0.04

E 197.2 ± 1.2 67 ±25 0.263 ± 0.023

Finally, we evaluated the validity of our devices as functional sensors by coating them with a R.H. sensing layer of cellulose acetate butyrate (CAB) of few microns thick. The coated devices were characterized and their signal is depicted in Fig. 3. Good reproducibility, time response (~66 seconds) and low hysteresis can be observed. Parylene-C acted as a moisture barrier for the substrate and helped to prevent condensation of water between electrodes at high R.H. too, making the devices very linear until 70% R.H. (Rz = 0.992) and stable. The absolute sensitivity of the devices resulted 5.46 ± 0.22 fF / 1% R.H.

(b) (C)

Fig. 2. (a) Optical pictures of the different spaced IDE structures presented in Table 1. The left (right) part in every case shows the first (first + second) printed comb; (b) Picture of a typical printed defect, (c) Example of bent printed devices, (d) Theoretical model, proofing that reducing the gap between IDE results in a great enhancement of the device capacitance value per surface area. Comparison with experimental values

85 „ 75

^ 55 E

-Relative Humidity [%] —►Capacitance [pF]

300 400 time [min]

g g 7.9

u u 7.8

Ü ™ 7.7 a g.

u 5 7-6 7.5

20 40 60 80 Relative humidity [%]

Fig. 3. (a) Capacitance value versus time for different R.H. steps, (b) Linear relation between capacitance and R.H. until 70 % R.H.

4. Conclusion

We successfully developed a novel strategy to fabricate printed IDE devices with reduced pitch and increased values of capacitance per area by the inclusion of a thin film of parylene-C interlayer. Devices with several different inter-electrodes gaps have been designed and fabricated with high yield by means of inkjet printing on PET substrate, demonstrating the feasibility of the method. An analytical model has been used to assess the potential of decreasing the inter-electrodes gap in enhancing the value of capacitance per surface area. The disagreement between model and experiments is explained by the low patter alignment accuracy allowed by the used printer. Finally, we demonstrated the potential of such IDE devices as well performing gas sensors by coating them with a test humidity sensing layer and characterizing their response against changes in R.H.

Acknowldegments

This work is funded by the EU FP7 project FlexSmell, a Marie-Curie Initial Training Network (ITN), under the Grant no. 238454. The authors thank Dr. Bill A. MacDonald from Dupont for supplying the plastic foils used as substrates.

References

[1] Derby B. Inkjet printing of functional and structural materials: fluid property requirements, feature stability, and resolution. Annu Rev MaterRes 2010;40:395-414.

[2] Molina-Lopez F, Briand D, de Rooij NF. All additive inkjet printed humidity sensors on plastic substrate. Sensors and Actuators B: Chemical 2012;166-167:212-222.

[3] Weremczuk J, Tarapata G, Jachowicz RS. The ink-jet printing humidity sorption sensor -modeling, design, technology and characterization. Meas Sei Technology!012;23:014003.

[4] Starke E, Türke A, Krause M, Fischer W-J. Flexible polymer humidty sensor fabricated by inkjet printing. Solid-State Sensors, Actuators and Microsystems Conf. (TRANSDUCERS) Int. 2011;June 2011, Beijing (China):l 152-5.

[5] Igreja R, Dias CJ. Analytical evaluation of the interdigital electrodes capacitance for a multi-layered structure. Sensors and Actuators A: Physical 2004;112(2-3):291-301.