Abstract
Measurement of 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 = Δ C Δ p p m × 100 ), the response time (tr), and the recovery time (tc) are determined and compared with a commercial SHAW, UKdew point meter. The fabricated double sided sensor has comparable sensitivity (S = 100%, tr (s) = 28, tc (s) = 40) with the commercial meter (S = 100.5%, tr (s) = 258) but has a faster response time. The proposed method of sensitivity enhancement is simple, and mass producible.
Highlights
Humidity sensors are widely used in different industrial, agricultural, medical, food preservation, home ventilation and air-conditioning (HVAC), and respiratory monitoring applications for the development of smart cities
The present work deals with the fabrication of capacitive porous alumina-based sensors with different sensing areas for sensing humidity in ppm
We have found that a moisture sensor with a larger effective area has higher sensitivity in a lower range
Summary
Humidity sensors are widely used in different industrial, agricultural, medical, food preservation, home ventilation and air-conditioning (HVAC), and respiratory monitoring applications for the development of smart cities. 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]. 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). When these two capacitors are connected in parallel, the sensitivity is enhanced almost two-fold but the actual size remains unchanged 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 sensor can be utilized for condition monitoring of electrical equipment and moisture measurement in human respiration
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