Reduction of Humidity Effect in WO₃ Thin Film-Based NO₂ Sensor Using Physiochemical Optimization

In this work, the effect of humidity on WO₃ thin films is studied. Two thin films of higher and lower oxidation states are characterized and studied for their interaction with water molecules on the metal oxide sensing surface. The film with higher oxidation is found to be more immune to humidity effect and hence, further used to reduce the same effect using absorbent filters and Nichrome deposited heater mesh. With the use of a Nichrome heater mesh, by increasing mesh temperature from 30 to 90 °C, there is a 63% reduction in absolute humidity at the sensor die. This in turn stabilizes the NO₂ sensor response, by decreasing the humidity dependent (40% RH to 90% RH) peak current variation for 3 ppm NO₂ exposure, from 65% to 26%. High-temperature ageing at a temperature of 90 °C and 90% RH is found to be effective in significantly reducing humidity effect on sensor current output and only 8.4% change in peak current is observed for 3 ppm NO₂ at variable humidity, which stabilizes the sensor current drift due to humidity variation.