High temperature sensors for monitoring accurate in-situ temperatures have been highly demanded in several industrial fields that require high temperatures. Surface acoustic wave (SAW) devices are widely used for high temperature sensors due to their advantages of being passive, operating without batteries, and being wireless. However, conventional SAW sensors cannot be applied for high temperature ranges above 400 °C due to thermal damage to the metallic interdigital transducers (IDTs) electrodes. Therefore, the development of suitable materials for the IDTs that can withstand high temperatures without thermally induced damage caused by the difference in thermal expansion coefficient (CTE) between film and substrate materials is necessary. Molybdenum (Mo) has attracted interest for IDTs due to its high melting point (2623 °C), high thermal conductivity (139 Wm-1K-1), low electrical resistivity (5.5 µΩ∙cm), and low CTE (4.8 x 10-6 at room temperature).

In the present study, Mo films with a thickness of 100 nm were deposited by DC magnetron sputtering on a Si substrate. AlN, SiO2 or combined SiO2/AlN films were deposited as barriers between Mo film and substrate and as cover layers on top of the Mo film with a thickness of 20 nm. The as-deposited films were annealed at 900 °C for 24 hours under UHV condition to annihilate defects and stabilize their microstructure. Then, the films were subjected to thermal cycling between room temperature and 900 °C. Meanwhile, the in-situ stress behaviours were monitored with a KSA multi-beam optical sensor (MOS) system. To investigate the stability of the film system in UHV conditions as well as in air, the films were annealed in air, and then the microstructure behaviour was analysed.