Integrated humidity and temperature sensing circuit fabricated by inkjet printing technology

Abstract

Due to the climate change and the modern living quality requirements, the demands on environmental monitoring devices, such as temperature and humidity sensors, increase fiercely in the recent decades. To meet the needs on those sensing devices, academic and industrial studies have utilized various materials to fabricate effective sensors to detect chemical materials under stringent requirements. Temperature and humidity sensors are widely used in industrial applications. Particularly, for processes sensitive to tiny environmental variations, such as micro-electronic fabrication process and green house for highly valuable plant growing, controlling temperature and humidity at a constant level is crucial to product quality. For commercial temperature and humidity control systems, there are two sensors with independent circuit to monitor temperature and humidity respectively. However, in extreme conditions, such as freezer or reflow ovens, these two sensors may interrupt each other and thus results in bad accuracy. In order to obtain precise temperature and humidity measurements, one can use a lookup table, or keeps humidity sensor at a constant temperature environment with an additional heating circuit. Unfortunately, in these ways, the real temperature and humidity conditions in the environment can never be detected simultaneously. Moreover, so far, there are still few research attempts to integrate the temperature and humidity sensors in the same circuit. In order to overcome this challenge, this research is aimed at printing sensing materials into one parallel circuit via a low cost direct writing technology. A layer-by-layer strategy is used to integrate the temperature-sensitive NiO layer and humidity-sensitive polyaniline (PANI) layer. These two elements are printed in parallel. The printed sensor circuit characteristics will be analyzed carefully to read temperature and humidity measurements at once. From the water adsorption on PANI thin film and temperature variation in NiO layer, the resistance and capacitance readings of the device in AC mode can be directly correlated to the environmental conditions. A correlation formula combining Arrhenius equation and parallel plate capacitance model will be developed to accurately describe the sensor responses.