Piezoelectric Micromachined Ultrasonic Transducers (PMUT)

Piezoelectric micromachined ultrasonic transducers (PMUTs) are MEMS-based piezoelectric ultrasonic transducers for acoustic imaging of the environment. Unlike solid piezoelectric transducers, which use the thickness motion of a plate made of a piezoelectric ceramic, PMUTs are based on the bending motion of a thin membrane coupled with a thin piezoelectric film.

Compared to solid piezoelectric ultrasonic transducers, PMUTs can offer several advantages, such as increased bandwidth, better support of flexible geometries, natural acoustic impedance matching to different media such as water, reduced voltage requirements also compared to CMUT, mixing of different resonant frequencies and integration potential with supporting electronic circuits, especially for miniaturised high-frequency applications.

PMUTs enable medical, endoscopic in vivo imaging devices and ultrasonic radar for autonomous mobility and industrial machinery. An array arrangement of PMUT sensors and actuators enables imaging of the environment.

At Fraunhofer IPMS we use piezoelectric or ferroelectric hafnium oxide (HfO₂). This ultra-thin ferroelectric transducer material was discovered in 2011 and is ideally suited for integration into existing CMOS and NEMS processes. The moderate piezoelectric coefficient and low dielectric constant of HfO2 is optimal for applications that require a combination of sensors or receivers and actuators or transmitters. The ultra-thin realisation of the NEMS sensors also enables a three-dimensional acquisition of the signal data. In addition to the conventional out-of-plane movement, miniaturised in-plane movements can also be realised, which can be used for miniaturised sensors.