Processing influence on the reliability of platinum thin films for MEMS applications

The influence of high temperature post-processes on the microstructural, electrical, and mechanical properties of evaporated platinum thin films with tantalum as adhesion layer was investigated. Post-processes, such as deposition of a silicon nitride passivation layer by LPCVD and annealing at high temperature (up to 830degreesC) in an inert or reactive gaseous atmosphere, were performed separately or successively on platinum/tantalum films having different thickness. An abnormal grain growth and a grain reorientation occurred in the platinum films during the heat treatments, with more severe hillocks formation when performed in air at temperatures higher than 500degreesC. Another parameter influencing the electrical characteristics (resistivity, temperature coefficient of resistance) of these films, in addition to the post-processing temperature and time, was the thickness of the platinum film. A passivation made of LPCVD silicon nitride film was shown to stabilise the electrical properties of the Pt/Ta films before post-processing or operating them at high temperatures, up to the deposition temperature of the passivation film. However, if the passivation is removed, post-processing of the Pt films at a higher temperature than 500degreesC in air induced more or less severe hillocks formation and adhesion problems could occur, related to the oxidation of tantalum. The adhesion of the Pt/Ta films was found to depend on the thickness of the LPCVD silicon nitride film deposited, which was removed before annealing in air. The Pt/Ta films properties depend on the annealing conditions, time and temperature, and on the history of the film, if the annealing was performed just after its deposition or after successive thermal processes.