Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications.

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments.

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.