Sensitivity enhancement of a PPM level capacitive moisture sensor

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 =_∆ppm ^∆C × 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.