Surface study of thin film gas sensors on a micro-machined substrate

The miniaturisation of gas sensors and the compatibility of their fabrication process with modern microelectronics technology are very important aspects of gas-sensing development. The reduction of power consumption could minimise not only thermal stresses and device degradation, but also enable to use battery operated sensors and smart sensor arrays. In the present work, variously surface-activated films of tin oxide deposited on a Si micro-machined substrate, which contained a Si3N4 membrane supporting the entire gas-sensing structure, have been investigated. Due to the very small thermal mass of the membrane, these devices can be operated in fast-pulsed temperature powering regime. The results of the measurements of gas sensitivity to aromatic hydrocarbons in fast-pulsed mode are presented and discussed. The gas sensitivity and device reliability are analysed on the basis of these results and the data of surface analysis of the sensors. The surface analysis of the samples has been carried out by using selected-area X-ray photoelectron spectroscopy (SA-XPS), SA-XPS depth profiling with cyclic Ar+ ion sputtering, scanning Auger microscopy (SAM) and scanning tunnelling microscopy (STM). These analytical techniques enable us to investigate the microchemical composition and morphology of the gas-sensing layers, both before and after the gas detecting cycles in real working conditions.