ppb level detection of NO₂ using a WO₃ thin film-based sensor: material optimization, device fabrication and packaging

In this study, we have investigated the thickness-dependent nitrogen dioxide (NO₂) sensing characteristics of a reactive-ion magnetron sputtered tungsten trioxide (WO₃) film, followed by morphological and electrical characterizations. Subsequently, the sensing material was integrated with an MEMS platform to develop a sensor chip to integrate with electronics for portable applications. Sputtered films are studied for their sensing performance under different operating conditions to discover the optimum thickness of the film for integrating it with a CMOS platform. The optimized film thickness of ∼85 nm shows the 16 ppb lower limit of detection and 39 ppb detection precision at the optimum 150 °C operating temperature. The film exhibits an extremely high sensor response [(R _g - R _a)/R _a × 100 = 26%] to a low (16 ppb) NO₂ concentration, which is a comparatively high response reported to date among reactively sputtered films. Moreover, this optimum film has a longer recovery time than others. Thus, an intentional temperature overshoot is made part of the sensing protocol to desorb the NO₂ species from the film surface, resulting in full recovery to the baseline without affecting the sensing material properties. Finally, the optimized film was successfully integrated on the sensor platform, which had a chip size of 1 mm², with an inbuilt micro-heater. The minimum power consumption of the microheater is ∼6.6 mW (∼150 °C), which is practically acceptable. Later, the sensor device was packaged on a Kovar heater for the detailed electrical and sensing characterizations. This study suggests that optimization of the sensing material and optimum operating temperature help to develop a highly sensitive, selective, stable, and portable gas sensor for indoor or outdoor applications.