This paper deals with the development of a novel metal-oxide-based sensor for measuring moisture in ppm level. The device consists of parallel plate capacitive structure having γ-Al ₂ O ₃ nanostructured sensing film deposited on the polyimide by sol-gel method. The pore morphology of the thin film has been studied by Brunauer-Emmett-Teller, atomic force microscopy, and field emission scanning electron microscopy. The sensor has been designed to make it suitable for integrated type of application. The electrical characteristics of the sensor have been measured using Agilent 4294A impedance analyzer over the frequency from 1 to 50 kHz. The sensitivity of the sensor for 3.5-800-ppm humidity range is 0.13 pF/ppm. The response and recovery time are 158 and 216 s, respectively. The experimental results show that the output is linear (4.2%), reproducible, and stable. Proposed work is aimed to develop a low power consumption sensor on fully flexible and plastic substrate commercially used in printed circuit board for mobile application.

Moisture measurement in ppm level is always challenging and costly. Among different technologies available for moisture measurement, the thin film capacitive sensor-based dew point meter offers a low-cost solution. Still, the cost of the dew point meter is high and it suffers from high response time. There is a need of wireless moisture measurement in ppm level that is not available with the commercial meter according to our knowledge. This paper deals with development of a low-cost passive wireless tag for capacitive type sensors. The tag can be energized by any 13.56-MHz commercial tag reader from a distance of 1 cm (approx). The tag is capable of transmitting sensor data at 2.4-GHz ISM band up to a distance of 12 m. An efficient algorithm has been developed to reduce the power consumption. The prototype system has been tested with two custom designed capacitive sensors to measure humidity over a wide range. Both sensors, having a highly stable hygroscopic film of Al₂O₃, are fabricated in the lab. Finally, a low-cost (approximately U.S. 250) digital hygrometer to measure trace moisture has been developed. The performances of the digital hygrometer have been compared with the commercial dew point meter, and accuracy is found to be nearly 1% in the range of 6.5-127-ppm moisture. The system can be employed for contactless measurement with any capacitive sensor in the range of 50-3200 pF.

A simple single chip integrated moisture sensor capable of wireless operation through inductive coupling has been proposed. The structure consists of planar inductor and parallel plate capacitor fabricated by sol-gel thin film of γ-Al₂O₃ (5 μm thick). The sensor chip is capable of measuring ppm level moisture in gaseous environment. The resonance frequency shift of the sensor in the presence of moisture excited by a high frequency ac signal is measured by impedance analyzer (4294A Agilent). The frequency sensitivity of the structure for a moisture range from 100 to 5000 ppm is nearly 119 Hz/ppm. The response and recovery times are 7 and 24 s, respectively. The experimental results show that the output is linear (4.2%), reproducible and stable. Device can be applied for wireless moisture measurement in ppm level.

Humidity sensing is a critical parameter in various applications. The commonly used capacitive humidity sensors in the literature have the interdigital planar electrode structure that may be treated as a fringing field capacitive (FFC) sensor. Several forms of the FFC sensors are reported for humidity measurement but comparative investigation of their response characteristics is rarely found. Therefore, this work presents a comparative analysis of planar capacitive sensor electrodes, for developing inexpensive humidity sensors. Five different electrode structures (i) Interdigitated (IDE) (ii) Serpentine (SRE) (iii) Meandered (MED) (iv) Spiral (SPE) and (v) Circular Cross (CCE) have been selected for comparative analysis. The design of electrodes has been done to obtain an equal effective sensing area keeping the track thickness, electrode gap, and sensing film fixed. The fringing field contribution for each electrode structure has been obtained through finite element analysis (FEA) in COMSOL. Then five FFC humidity sensors are fabricated with Polyethylene glycol hydrophilic sensing film. The sensors are exposed to different humidity levels (8-80 %RH) to obtain their static and dynamic parameters using Mixing Flow and Saturated Salt Solution methods. The comparison has been made on fringing field capacitance, sensitivity, response/recovery time and influence of oxygen. The sensors with IDE electrodes show the highest sensitivity with a lesser fringing field effect whereas the circular cross electrode has the better recovery with the highest fringing effect contributing 60% of its total capacitance value. However, the choice of electrode structure for fabricating the sensor will depend on its specific applications and requirements.