Optimization of a Wafer-Level Process for the Fabrication of Highly Reproducible Thin-Film MOX Sensors

Thin-film Metal Oxide Semiconductor (MOX) gas sensors are characterized by high sensitivity and fast response. Those characteristics make them very promising among the several existing technologies for the production of solid state gas sensors. Furthermore, by means of Silicon micro-machining technology, MOX sensors can be made on micro hotplates allowing to reach very low power consumption and the batch production guaranties a high yield. However, reproducibility and reliability are still major issues preventing the use of thin-film MOX sensors in mass-market applications. In this work a wafer level fabrication process for micro-machined low power consumption thin film MOX sensor arrays is reported. Different solutions for the optimization of the fabrication process are investigated, aiming to increase the reproducibility. The critical technological steps related to signal generation and acquisition, like the thin-film definition and positioning and the definition of the sensing layer electrodes, have been optimized. The devices considered are 4-sensor arrays based on thin films of SnO2 deposited by a modified RGTO technique on micro-machined low-power hotplates. The different fabrication techniques are described in detail. 45 sensors from 3 wafers, made using the different fabrication techniques, are comparatively characterized. The spread of the main sensor functional parameters values show an evident decrease when the optimized fabrication process is used.