Processing influence on the reliability of platinum thin films for MEMS applications
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Processing influence on the reliability of platinum thin films for MEMS applications Danick Briand, Stephan Heimgartner*, Mireille Leboeuf, Massoud Dadras, and Nico F. de Rooij Institute of Microtechnology, University of Neuchâtel Rue Jaquet-Droz 1, P.O. Box 3, CH-2007 Neuchâtel, Switzerland * currently at elmicron AG, Sachseln, Switzerland ABSTRACT 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 830°C) 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 500°C. 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, postprocessing of the Pt films at a higher temperature than 500°C 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.
INTRODUCTION Platinum is a material used in microsystems due to its resistance to oxidation and its high melting point, which offers it the capability of withstanding high temperature post-processing and operation. One main application of platinum is as a heater element such as in flow and gas sensors, and in chemical micro-reactors. The influence of high temperature on platinum has been previously studied, mainly for titanium as an adhesion layer [1-5]. The work that has been reported so far on the Pt/Ta thin films stability was performed to obtain stable electrodes for ferroelectric thin films [6] or concentrated on electrical characterisations which were performed at high temperature [7]. In [7], some guidelines were given to increase the reliability of such films at high temperatures but little was said on the influence of post-processing on the electrical and mechanical characteristics of these films. The aim of this work was to define the postprocessing parame
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