Capacitive ppm moisture sensor with simple method of improving sensitivity

Capacitive ppm moisture sensor with simple method of improving sensitivity: History Edit

Subjects: Engineering, Electrical & Electronic

Presence of moisture even in ppm level in different gases used for different applications plays important role on the quality of products measurement in different gases for industrial moisture at ppm or ppb level is very difficult and the fabrication of such sensors at low cost is always challenging. High sensitivity is an important parameter for trace level (ppm) humidity sensors. Anelectronic detection circuit for interfacing the humidity sensor with high sensitivity requires a simple hardware circuit with few active devices. The recent trends for increasing the sensitivity include fabricating nanoporous film with a very large surface area. In the present work, the sensitivity of a parallel plate capacitive type sensor with metal oxide sensing film has been significantly improved with an aim to detect moisture from 3 to 100 ppm in the industrial process gases used to fabricate semiconductors and other sensitive electronic devices. The sensitivity has been increased by (i) fabricating a nanoporous film of aluminum oxide using the sol-gel method and (ii) increasing the cross-sectional area of a parallel plate capacitor. A novel double sided capacitive structure has been proposed where two capacitors have been fabricated—one on the top and one on the bottom side of a flat alumina substrate—and then the capacitors are connected in parallel. The structure has twice the sensitivity of a single sensor in the same ppm range but the size of the structure remains unchanged. The important characteristics of the sensors such as the sensitivity (S = $\frac{Δ C}{Δ p p m} \times 100$ ), the response time (t_r), and the recovery time (t_c) are determined and compared with a commercial SHAW, UKdew point meter. The fabricated double sided sensor has comparable sensitivity (S = 100%, t_r (s) = 28, t_c (s) = 40) with the commercial meter (S = 100.5%, t_r (s) = 258) but has a faster response time. The proposed method of sensitivity enhancement is simple, and mass producible

1. Introduction

Moisture measurement at ppm level is essential for condition monitoring of gas insulated substations (GISs), transformers, and circuit breakers in order to achieve uninterrupted power supply in various applications including smart cities. The sensor should be sensitive to sudden step change in moisture and available at low cost with a relatively long calibration period. The dynamic range of humidity measurement, which extends from %RH to ppb level, is extremely large [1,2,3,4,5]. Capacitive sensors are widely used for the measurement of humidity over a wide range. The most widely used capacitive sensor in %RH is the interdigitated electrode structure. The interdigitated sensor, to the best of our knowledge, is only suitable for RH level humidity measurement [5]. Very recently, an attempt has been made by the authors to measure moisture at ppm level by a micro interdigitated capacitive sensor. However, the sensor does not show sensitivity less than 200 ppm [6]. A large number of research articles is published every year in different journals on the topic of %RH humidity measurement. However, the number of research papers for ppm level moisture detection is very low [5]. The cost of a commercial ppm level moisture sensor is at least fifty times higher than that of RH sensors [6,7].

Aluminum oxide may be suitable for trace level moisture measurement, since it is thermally stable, and chemically inert in a corrosive environment. Among the different methods of alumina film preparation, the sol-gel method is a simple, less expensive method to prepare a pure ceramic thin film of desired thickness. The film can be formed on the ceramic substrate by the dip coating, spin coating or spray coating method [1,2,6,7,8,9,10]. Incomparison to other ceramic materials, aluminum oxide (Al₂O₃) is highly hydrophilic and it is one of the most useful materials for moisture measurement in most industrial gases [4,10,11]. Several works have been reported in the recent past to measure moisture in the range of 0–100 ppm. However, enhancement of the sensitivity is still required to make a prototype system for the possibility of commercial applications. In addition, there is a need to increase the dynamic range below 10 ppm [8,9,12,13,14,15,16,17,18,19,20,21,22,23]. Continuous efforts are made by the researchers to fabricate highly porous nanostructures such as nanowire, nanotubes, and nanopores using different fabrication techniques. Such nanostructure films result in a very high surface area suitable for humidity sensing [23,24,25,26,27,28,29]. Efforts are also being made to explore new fabrication techniques and materials to achieve the goal of high sensitivity and selectivity [30]. The methods followed for the fabrication of nanostructures with desirable pore morphology are costly, time consuming and very often lead to failure. It is not necessary that the sensitivity will increase by increasing the surface area. It also requires tuning the pore morphology. Most recently, the sensitivity of a capacitive trace moisture sensor was enhanced by increasing the surface area of a pure alumina nanoporous film with the addition of polymer polyethylene glycol (PEG). The surface area of the alumina is increased two-fold but the sensitivity is increased by approximately 61%. It is found that with the increase in surface area, the size of the pores also increases [8].

In the present work, we have studied the sensitivity of a capacitive trace moisture sensor for measuring moisture in ppm. The sensitivity has been addressed by (i) selecting a suitable pore morphology nanostructured thin film of Al₂O₃; and (ii) increasing the cross-sectional area of the parallel plate electrode. To enhance sensitivity further, a unique structure with two identical parallel plate capacitors on each side of the alumina substrate has been fabricated. When these two capacitors are connected in parallel, the sensitivity is enhanced almost two-fold but the actual size remains unchanged. Also, this structure prevents the wastage of very useful dip-coated film on the opposite side of the substrate as reported previously in [7,9,14,16,17,21,22,23]. The proposed method of sensitivity enhancement is easy, simple, and less time consuming. The sensor can be utilized for condition monitoring of electrical equipment and moisture measurement in human respiration.