Performing high-temperature measurements on the rotating parts of aero-engine systems requires wireless passive sensors. Surface acoustic wave (SAW) sensors can measure high temperatures wirelessly, making them ideal for extreme situations where wired sensors are not applicable. This study reports a new SAW temperature sensor based on a langasite (LGS) substrate that can perform measurements in environments with temperatures as high as 1300 °C. The Pt electrode and LGS substrate were protected by an AlN passivation layer deposited via a pulsed laser, thereby improving the crystallization quality of the Pt film, with the function and stability of the SAW device guaranteed at 1100 °C. The linear relationship between the resonant frequency and temperature is verified by various high-temperature radio-frequency (RF) tests. Changes in sample microstructure before and after high-temperature exposure are analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The analysis confirms that the proposed AlN/Pt/Cr thin-film electrode has great application potential in high-temperature SAW sensors.
We proposed a novel Langasite (LGS) bonding method only using high temperature to solve the manufacturing difficulty of the sealed microcavity of pressure sensors. The optimal bonding parameters by comparative experiments were defined as 1350 °C for 3 h. Due to simple experimental conditions, low experimental cost, and be suitable for bonding wafers with various sizes, the method is convenient for popularization and mass-production, thus promoting the development of surface acoustic wave (SAW) devices at high temperatures. Simultaneously, an intact microcavity was observed by scanning electron microscopy, and a tight and void-free bonding interface with a transition layer thickness of 2.2 nm was confirmed via transmission electron microscopy. The results of tensile and leakage experiments indicated that the bonded wafer with the sealed microcavity exhibited a high bonding strength of 4.02 MPa and excellent seal performance. Compared to the original wafer, the piezoelectric constant of the LGS bonded wafer had a reduction of only 4.43%. The above characteristics show that the sealed microcavity prepared by this method satisfies the conditions for fabricating the LGS SAW pressure sensors. Additionally, based on the bonding interface characterizations, the mechanism of LGS bonding has been investigated for the first time.
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