This paper reports on a significant further improvement of the high temperature stability of RuAl thin films (110 nm) on the piezoelectric Ca 3 TaGa 3 Si 2 O 14 (CTGS) and La 3 Ga 5 SiO 14 (LGS) substrates. RuAl thin films with AlN or SiO 2 cover layers and barriers to the substrate (each 20 nm), as well as a combination of both were prepared on thermally oxidized Si substrates, which serve as a reference for fundamental studies, and the piezoelectric CTGS, as well as LGS substrates. In some films, additional Al layers were added. To study their high temperature stability, the samples were annealed in air and in high vacuum up to 900 °C, and subsequently their cross-sections, phase formation, film chemistry, and electrical resistivity were analyzed. It was shown that on thermally oxidized Si substrates, all films were stable after annealing in air up to 800 °C and in high vacuum up to 900 °C. The high temperature stability of RuAl thin films on CTGS substrates was improved up to 900 °C in high vacuum by the application of a combined AlN/SiO 2 barrier layer and up to 800 °C in air using a SiO 2 barrier. On LGS, the films were only stable up to 600 °C in air; however, a single SiO 2 barrier layer was sufficient to prevent oxidation during annealing at 900 °C in high vacuum.
Keywords: CTGS; LGS; RuAl; SAW sensors; high temperature stability; interdigital transducer material; thin films.