Single-cell system using monolithic PMUTs-on-CMOS to monitor fluid hydrodynamic properties

In this work, a single cell capable of monitoring fluid density, viscosity, sound velocity, and compressibility with a compact and small design is presented. The fluid measurement system is formed by a two-port AlScN piezoelectric micromachined ultrasonic transducer (PMUT) with an 80 μm length monolithically fabricated with a 130 nm complementary metal-oxide semiconductor (CMOS) process. The electrode configuration allows the entire system to be implemented in a single device, where one electrode is used as an input and the other as an output. Experimental verification was carried out by exploiting the features of piezoelectric devices such as resonators and acoustic transducers, where a frequency shift and amplitude variation are expected because of a change in density and viscosity. A sensitivity of 482 ± 14 Hz/kg/m³ demonstrates the potential of the system compared to other dual-electrode PMUTs. In addition, according to the acoustic measurement, the sound velocity, fluid compressibility, and viscosity coefficient can be extracted, which, to the best of our knowledge, is novel in these PMUT systems.