Preparation And Research Of High Temperature Surface Acoustic Wave Resonators Based On Multilayer Composite Film Electrode

Traditional sensors are prone to performance failures in extremely high-temperature industrial environments,including health monitoring systems for aerospace engine parts,petroleum extraction,chemical production industries,and engine and emission systems in automobiles and ships.High-temperature surface acoustic wave（SAW）sensors are specifically designed to withstand extreme temperatures,have high reliability and durability,and their advantages of wireless,passive,low power consumption,and small size make them suitable for high-temperature rotation,power consumption or limited space scenarios.With the continuous increase in application scenarios,higher requirements are put forward for stable monitoring of SAW devices in high-temperature harsh environments.The main challenge in maintaining the high-temperature stability of SAW devices is to improve the high-temperature resistance of the interdigital transducer（IDT）electrodes.This thesis is based on platinum-rhodium（Pt-Rh）alloy,designed and prepared various multi-layer composite thin film electrodes with different structures,and further studied the highest operating temperature and thermal stability of the electrodes.Subsequently,the structure of the multi-layer composite thin film electrode was applied to IDT,and SAW devices were successfully fabricated and subjected to high-temperature heat treatment testing.The specific work content is summarized as follows:（1）Material selection and structural design of alloy layers,pinning material layers,and barrier layers in multilayer composite thin film electrodes were carried out,and the electrode fabrication process and maximum operating temperature testing were completed.The study showed that increasing the thickness of the thin film electrode improved its stability and effectively suppressed electrode agglomeration at high temperatures.In addition,it was found that increasing the number of pinning material layers could also improve the high-temperature performance of the electrode,as the pinning material doped into the alloy layer could enhance the stability of the thin film electrode’s grain boundary structure.Based on this,electrode structures with various protective layers were fabricated and tested.A passivation protective layer can reduce the contact between the electrode surface and air and make the electrode agglomeration require more energy,thereby improving the maximum operating temperature of the electrode.It is noteworthy that the designed Zr O₂/Pt-Rh/Al₂O₃/Pt-Rh/Al₂O₃ electrode structure can reach a maximum operating temperature of 1600℃in air.（2）The thermal stability of the multilayer composite thin film electrode was tested in air.The Zr O₂/Pt-Rh/Al₂O₃/Pt-Rh/Al₂O₃ thin film electrodes were subjected to 5 cycles of 1-hour heat treatment at 1200℃and 2 cycles of 1-hour heat treatment at 1400℃,and both showing significant stability.When the experimental temperature was increased to 1500℃,the electrode could operate stably for at least 30 minutes.The agglomeration process of the electrode after heat treatment was summarized into three stages:formation of holes,growth of holes,and contact between adjacent holes to form isolated particles,as observed by scanning electron microscopy（SEM）.In addition,to suppress the oxidation reaction of metals at high temperatures,a testing scheme in a nitrogen environment was proposed.This effectively improved the high-temperature performance of the Zr O₂/Pt-Rh/Al₂O₃/Pt-Rh/Al₂O₃ electrode structure,and its maximum operating temperature was increased to above 1650℃.After being subjected to 1 hour heat treatment at 1600℃,it still maintained good stability.Large-area thin films on a PAD also show significant agglomeration and degradation at high temperatures.In order to ensure that the bond between the device and the wire is not affected during high-temperature testing,the degradation behavior of the large-area Pt-Rh/Al₂O₃/Pt-Rh/Al₂O₃ multilayer composite thin film was observed at high temperatures.The Pt-Rh composite thin film remained intact and conductive even after being kept at 1600℃for 1 hour in both air and nitrogen environments.（3）The structure of Al₂O₃/Pt-Rh/Al₂O₃/Pt-Rh/Al₂O₃ was applied to the IDT electrodes of SAW devices,and the simulation analysis was carried out using COMSOL software.Based on an Al N piezoelectric substrate,a high-performance single-port resonator type SAW device was designed and fabricated.Furthermore,the SAW device was subjected to high-temperature testing.The experimental results showed that the SAW device exhibited good stability after being kept at 1200℃in air for 1 hour and at 1400℃in a nitrogen environment for 1 hour,respectively.