Temperature and strain SAW/BAW sensors on metallic substrates with RFID capability

Surface acoustic wave (SAW) strain sensors require an adhesive—typically a glue—for the mounting on the part under measurement. This comes with strain-transfer, reproducibility and aging issues. In this paper, we propose a novel glue-less solution where the SAW sensor is directly fabricated onto the surface of interest, here a metallic substrate. Here, we study the layered structure ZnO/Ti numerically and experimentally, with ZnO as the piezoelectric layer and titanium as the substrate. In this structure, both bulk acoustic waves and SAW can propagate, and we used both of them to monitor temperature variations up to 300 °C and strain levels up to 1800 μϵ. Moreover, reflective delay line (R-DL) designs were used, to provide the future users with radio frequency identification functionalities. In order to overcome the limitations due to the relatively low electromechanical coupling coefficient of ZnO/Ti, a specific R-DL layout with connected inter-digital transducers was used. The obtained experimental results confirm that the proposed glue-less R-DL structure is a promising solution for the independent evaluation of temperature and strain, with identification.