Novel narrowband acoustic sensors for sub-GHz wireless measurements

High-overtone Bulk Acoustic Resonator is an acoustic transducer based on an excitation of a bulk acoustic wave by a thin piezoelectric film bonded to a thick low acoustic loss substrate. This combination of materials aims at providing on the one hand a high frequency transducer as defined by the thickness of the thin piezoelectric layer, and on the other hand the robustness of a thick substrate while keeping the acoustic properties of single crystal piezoelectric materials. More specifically, this architecture provides high quality factors using bulk acoustic wave at frequencies only accessible to surface acoustic wave (SAW) devices with interdigitated transducer generation. The multimode spectrum is well suited for an openloop, wireless interrogation strategy in which the frequency of the incoming electromagnetic wave defines the operating point. We here demonstrate the use of a frequency sweep RADAR-like network analyzer for probing through a wireless link HBARs with different temperature coefficients in order to perform temperature measurements insensitive to other correlated noise sources (capacitive frequency pulling, electrode aging, stress).