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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

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Recent studies have evidenced that Pt/AlN/Sapphire surface acoustic wave (SAW) devices are promising for high-temperature high-frequency applications. However, they cannot be used above 700°C in air atmosphere as the Pt interdigital transducers (IDTs) agglomerate and the AlN layer oxidizes in such conditions. In this paper, we explore the possibility to use an AlN protective overlayer to concurrently hinder these phenomena. To do so, AlN/IDT/AlN/Sapphire heterostructures undergo successive annealing steps from 800°C to 1000°C in air atmosphere. The impact of each step on the morphology, microstructure, and phase composition of AlN and Pt films is evaluated using optical microscopy, scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS). Finally, acoustical performance at room temperature of both protected and unprotected SAW devices are compared, as well as the effects of annealing on these performance. These investigations show that the use of an overlayer is one possible solution to strongly hinder the Pt IDTs agglomeration up to 1000°C. Moreover, AlN/IDT/AlN/Sapphire SAW heterostructures show promising performances in terms of stability up to 800°C. At higher temperatures, the oxidation of AlN is more intense and makes it inappropriate to be used as a protective layer.

In this work, the performance of AlN/Sapphire structure in high frequencies is investigated. Several SAW devices were fabricated with various designs (free-space delay, wavelength, metallization ratio, …) to study simultaneously different parameters (acoustic velocity, electromechanical coupling (K²), acoustic propagation loss (α), TCF) versus frequency and temperature. Experimental results showed that as expected, α increases with temperature while K² is enhanced at high temperatures. Due to the antagonistic evolution of these two parameters, insertion loss decreases or increases as function of the free-space delay. We also demonstrated that this structure allows fabrication of devices operating up to 1.5 GHz and that the frequency varies linearly with temperature.

The possibility to generate simultaneously Surface Acoustic Wave (SAW) and Waveguiding layer acoustic wave (WLAW) in layered structures AlN/ZnO/Silicon was investigated. A delay line operating at 525 MHz was tested versus temperature in air and in contact with liquid. Experimental characterizations were also supported by modeling using the commercial software (COMSOL Multiphysics). The full delay line was simulated and liquid modeled by an additional layer on the top of AlN film.