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

摘要

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 1000u000b0;C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000u000b0;C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700u000b0;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 500u000b0;C and 300u000b0;C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around –75 ppm/u000b0;C. Between room temperature and 300u000b0;C, both electromechanical coupling coefficient K2 and propagation losses increase, so the evolution of delay lines'' insertion losses with temperature strongly depends on the length of the propagation path.