Theoretical examination of thermo-migration in novel platinum microheaters

Abstract

Conductometric gas sensors based on SMO films must be heated to temperatures up to 550 °C in order to initiate the molecular adsorption process at the SMO film's surface. Very often platinum is used as the microheater material. The long-term reliability of these devices are primarily associated with the mechanical stability of the micro-electro-mechanical systems (MEMS) structures used to hold the microheater suspended and thermally isolated from other integrated components, such as analog and digital circuitry. However, previous studies have shown that that electro-migration and thermo-migration phenomena could potentially exacerbate the stress build-up in platinum microheaters and contribute to their eventual failure. In this manuscript we propose a means to quantify the impact of vacancy transport on stress build-up in two novel microheater designs under electro-migration and thermo-migration phenomena. The first design is aimed at improved temperature uniformity and the second is aimed at microheater array operation, taking advantage of high temperature gradients to simultaneously provide multiple temperatures at different sensor locations. Our analysis shows that the thermo-migration force is much higher than the electro-migration force, meaning that the high thermal gradients in these devices contribute far more to vacancy transport than the atom transport induced by the electron wind. Furthermore, we calculate that our proposed designs are highly resistant to failure due to vacancy migration by calculating the mean-time-to-failure to be on the order of 1015 seconds under typical operating conditions.