Removal of Stress Hillocks from Platinum-Alumina Electrodes Used in High-temperature SAW Devices

Microwave acoustic devices operating at elevated temperatures, such as surface acoustic wave (SAW) duplexers or high-temperature sensors, require stable thin film electrodes capable of operating in such conditions. Stresses at the interface between the substrate and the thin film electrodes originated during electrode deposition and/or during device operation can foster film agglomeration, formation of whiskers, hillocks, electromigration, separation of the metal and oxide in composite electrodes, which degrade the electrode conductivity and the device's long-term stability and performance. Thin film composite Pt-Al₂O₃ has been successfully used as electrodes in langasite (LGS) SAW devices and sensors up to 900°C, but the formation of hillocks due to stress is seen as a limitation to the device long-term stability. In this work, a modified graded film deposition technique has been implemented targeting the release of film stress between the substrate and the thin film composite electrode due to fabrication and exposure to high-temperature. The technique successfully mitigated the formation of hillocks and material agglomeration as revealed by the SEM images obtained, and can be easily extended to other metallic or composite thin films.