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Piezoelectric aluminum nitride films were deposited onto 3 in. ͓0001͔ sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly ͑002͒-textured AlN films to be used in surface acoustic wave ͑SAW͒ devices for high temperature applications. The synthesized films, typically 1 m thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction ͑XRD͒, transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly ͑002͒-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3°can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250°C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800°C for 90 min. The frequency response, recorded after the annealing process, confirmed that the thin films were still strong in a high temperature environment and that they had retained their piezoelectric properties.

International audienceThe achievement of surface acoustic wave (SAW) devices stable in high-temperature oxidizing atmospheres requires the development of conductive thin film electrodes that can withstand such harsh conditions. Recent studies have demonstrated the suitability of Pt-based alloys, multilayers or nanocomposite films for temperatures up to 800 °C. Electrodes based on new materials and structures still have to be developed for applications taking place at higher temperatures. In this perspective, thin films based on iridium could be good candidates regarding the high melting point, and thus the low diffusion coefficients of this noble metal. In particular, Ir-Rh bulk alloys have shown superior performance as spark plug electrodes, which have to resist concurrently to physical and chemical wear such as high-temperature SAW electrodes. Consequently, this paper deals with the high-temperature behavior of Ir-Rh thin films. Ir-Rh alloys and multilayers films, with an Ir atomic ratio between 10 and 50%, are deposited by one-gun electron beam evaporation method. The impact on the films of a 4-days annealing treatment at 800 °C in air is studied by X-ray diffraction, scanning and transmission electron microscopy, electron energy loss spectroscopy and four-points probe resistivity measurements. It turns out that all the films oxidized during the annealing period. The post-annealing electrical properties are highly dependent of the initial composition of the film: the higher is the Ir rate in the film, the lower is the electrical resistivity after annealing. Moreover, an Rh2O3 overlayer, with a thickness of some tens of nanometers, forms at the surface of the film, confirming previous observations made on Ir-Rh bulk alloys. First SAW measurements made on devices based on Ir30Rh70 alloy electrodes are very promising as a SAW signal is still clearly visible after the 4-days annealing process, while no agglomeration phenomenon can be observed