Multi-DoF AlN-on-SOI BAW MEMS resonators with coated ZIF-8 for gas sensing application.

This paper explored the practical utility of gas sensing applications based on the multi-degree-of-freedom (Multi-DoF) bulk acoustic wave (BAW) resonant sensors, including 1, 2, and 3-DoF devices, where piezoelectric actuation and sensing methods were adopted. Zeolitic imidazolate framework-8 (ZIF-8) was chosen for the adsorption and desorption of the ethanol vapor, thereby facilitating the gas sensing mechanism and introducing the external mass changes to the multi-DoF resonating system. Similar to conventional quartz crystal microbalance (QCM) gas sensors, the frequency shift of all the devices (1, 2, and 3-DoF devices) was tracked to characterize the sensitivity. Besides, for the 2 and 3-DoF devices, the amplitude ratio (AR) change was also recorded and observed with an enhancement in performance. Compared with the state-of-the-art gas sensor based on 2-DoFcapacitively coupled resonators, the presented devices achieved better Q factor in air, stability, and resolution in terms of both frequency shifts and AR changes. The dominant mass change (dominant stiffness change in the state-of-the-art) of the proposed resonant devices matched well with the theoretical mass sensing principle, which is both predictable and crucial for the accurate modeling of the practical mass sensor. Furthermore, a lower ethanol vapor concentration from 0.1% to 2% was successfully detected by the proposed 2-DoF device, demonstrating even better sensing performance than that of the state-of-the-art.