High-Overtone Bulk Acoustic Resonator (HBAR) as Passive Sensor: Towards Microwave Wireless Interrogation

Increasing the operating frequency of wireless passive sensors is suitable for reducing the global sensor size currently dominated by antenna dimensions, and for improved directivity of the probe electromagnetic signal with either electronic or mechanical beam sweeping by the reader for space domain multiplexing. While surface acoustic wave (SAW) transducers patterned on piezoelectric substrate are one of the standard approach for passive wireless sensing, their operating frequency is limited by lithography resolution. One alternative approach to increase the operating frequency is to exploit bulk acoustic resonators operating at high overtone -- the so-called HBARs. Furthermore, In this paper, the interrogation of such devices is demonstrated either in the time domain (delay line) or frequency domain (resonator), taking advantage of their spectral characteristics, i.e. a comb of modes in the frequency domain, or a comb of time-domain reflections.