Printed electrodes structures as capacitive humidity sensors: A comparison

Highlights

• Four different electrodes designs are compared in terms of their feature as capacitive planar sensors.

• Small, low-cost and flexible capacitive humidity sensors were designed using COMSOL Multiphysics.

• The sensors were fabricated by inkjet-printing on flexible substrate without extra sensing layer and fully characterized.

• The advantages and disadvantages of each studied layout are remarked.

Abstract

This work discusses about four planar printed capacitive sensors with different geometrical layouts, fabricated by inkjet printing on a flexible substrate and used as humidity sensors. In particular, we show a comparison among interdigitated electrodes, meandered electrodes, spiral electrodes, and serpentine electrodes in terms of fabrication yields, sensitivities to relative humidity as well as thermal drift taking into account frequency dependencies. In addition, numerical simulations have been performed to further investigate the characteristics of these sensors. All sensors present similar behavior in frequency with humidity. Taking into account sensitivity within the same sensing area, the highest value is achieved by serpentine electrodes, followed by spiral electrodes, interdigitated and meandered electrodes, in this order. The best configuration will be dependent on the specific application and its requirements.

Introduction

Capacitive structures are widely used in electronics to address many different applications [1]; they are especially interesting in the field of sensors due to their characteristics, such as low energy consumption, non-intrusive and non-invasive, no radiation and fast response [2], [3]. The most common sensing capacitive structures are the parallel plate (PP) and the interdigitated electrode (IDE). PP is characterized by the simplicity of its geometry and the ease of calculation and modelling.

IDE sensors are a particular case of planar capacitive structures where the sensor electrodes are placed in a co-planar plane [4]. The planar structure allows to access the device from only one side [5], which is particularly useful when the access to an material under test (MUT) is limited or the other side should be open to the ambient. The advantage of this kind of structures is the fact that they can be fabricated on a substrate by deposition of a layer without including any step of micromachining. That allows compatibility with any kind of technology. Furthermore, this geometry has been used to fabricate with multiple materials and following different manufacturing process, from integration in semiconductor dices to printing on flexible substrates [6], [7], [8]. This additional feature makes planar capacitive sensors an attractive option for applications in material characterization [5], non-destructive testing (NDT) [9], proximity/displacement measurement [10], intelligent human interfacing [11], and imaging [12], [13]. Many efforts have been devoted to the theoretical modelling of the planar capacitive structures. Igreja and Dias [14] presented a theoretical model of the capacitance of IDE structure. These capacitors have also been simulated using different simulations tools [15], [16], [17]. Some authors have analysed other designs such as spiral electrodes and concentric rings in order to improve the performance of this design [7], [18], [19]. Other structures such as rectangular-shaped [20] and comb-shaped sensor arrays [21] were studied, showing that desired linearity and sensitivity can be achieved by the optimal selection of a set of structural parameters. Zeothout et al. [22] analysed a rectangular shaped planar sensor using a numerical method and also compared its performance according to different material properties and boundary conditions.

On the other hand, flexible electronic devices manufactured by printing techniques have become increasingly attractive thanks to their feasibility of large scale processing, potential low-cost per surface area and mechanical flexibility. Great advances have been achieved in the design of flexible and printed humidity sensors [23], [24], [25], [26] as well as sensors for other gases and vapours [27], [28], [29]. The classical transduction mechanism of these humidity sensors is capacitive due to the requirement of low energy consumption, in particular, through changes in the electrical permittivity of some structural layer of the capacitor or the dielectric thickness. Different printed techniques have been used to manufacture capacitive printed devices and different approaches have been followed to add the sensing capability into the capacitor. The most common strategy has been to deposit a sensing layer over the electrodes [16], [30], [31] such as cellulose acetate butyrate (CAB), poly(methyl methacrylate) (PMMA) or polyvinylchloride (PVC), among others. Another possibility has been to select a flexible substrate as sensing element saving fabrication steps, in this sense, polyimide [26] and photographic paper [25] have already been used. One of the main interfering factors to obtain an accurate humidity measurement is temperature. In order to compensate this dependence, several strategies have been already described such as differential measurements with reference capacitors (not sensitive to humidity) [32], including additional temperature sensors [33], [34] or using a sensitive layer with very low thermal drift [35], [36]. This latter alternative does not require additional devices, and therefore, less area and energy is consumed.

In this work, we will show the design, fabrication and characterization as humidity sensor of four different coplanar electrodes comparing and contrasting their characteristics. In this regard, we present the design, fabrication and characterization of capacitive humidity sensors which uses the flexible substrate as sensitive element. These capacitors have been printed with silver nanoparticles ink by inkjet-printing on a polyimide thin film. In the context, this paper discusses planar printed capacitive sensors in terms of fabrication yields, sensitivity to relative humidity as well as thermal drift taking into account frequency dependencies. Further investigations of the sensor designs have been carried out using a numerical method. The differences among these planar capacitive sensors are pointed out as well as their advantages and disadvantages of the different electrodes designs.