High temperature degradation of Pt/Ti electrodes in micro-hotplate gas sensors

Abstract

This paper examines the high temperature degradation of Pt–Ti thin films which configure the electrodes of a micro-hotplate designed in agreement with those used for gas sensor applications. FEM simulations of the micro-hotplate have been combined with atomic force microscopy (AFM) and Auger electron spectroscopy (EAS) measurements. Experimentally, the main mechanical failure mechanisms that have been identified are hillocks and delaminations detected in the Pt–Ti micro-electrodes produced by the high thermo-mechanical stresses induced during the operation of the micro-hotplate. Additionally, concurrent with the annealing-induced Pt microstructure changes, Ti from the adhesion layer and N from the Si3N4, migrated into the Pt film. It was shown with AES that, for high temperature operations, the titanium and nitrogen reach the Pt surface. Mechanical failures affect the contact between electrodes and the gas sensing material, and the Ti and N diffusion modifies the electrodes electrical resistivity. It is expected that both kinds of effects alter the electrical connection between the sensing layer and the electrodes, thus influencing the operation of the whole gas sensor.