Gas sensing properties of metal oxide nanowires and their CMOS integration

Abstract

Highly sensitive gas detecting devices are of significant importance in various applications such as environmental air quality monitoring or occupational safety and health. In the last decades considerable advances have been made in the development of conductometric metal oxide gas sensors and in optimization of sensor performance regarding sensitivity and selectivity. Traditional metal oxide based gas sensors employ thin or thick film resistors and their working principle is based on electrical conductance changes due to interaction with surrounding gas molecules. Due to favorable material properties, nanowire-based devices are considered as potential candidate for the realization of miniaturized, next-generation gas sensors with improved performance. In particular the integration of nanomaterials with CMOS technology is expected to result in low power consumption gas sensor systems for daily life applications. In this thesis, the gas sensing properties of nanowire devices based on two different metal oxide materials, i.e. cupric oxide (CuO) and zinc oxide (ZnO), are investigated. CuO and ZnO nanowires are obtained by various synthesis techniques on wire and thin substrates. Complimentary characterization methods are employed for structural as well as compositional analysis. Fabrication processes are developed for the realization of single nanowire as well as nanowire array devices, which are characterized in terms of electrical properties and gas sensing performance. The nanowire gas sensors are operated at elevated temperatures up to 350 _C and device resistance changes are investigated during exposure to water vapor and small concentrations of the toxic gases CO (ppm-level range) and H2S (ppb-level range). The results obtained for the different sensor configurations are compared and interpreted in terms of sensing mechanism model. Furthermore, the integration of CuO nanowire arrays on CMOS microhotplates is demonstrated resulting in miniaturized, low power consumption gas sensor devices.