Ferroelectric micro-machined ultrasonic transducers for biomedical and processing-in-sensor applications

Abstract

Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) are Micro Electro-Mechanical Systems (MEMS) devices that have become an established technology in applications such as rangefinding, fingerprint sensing and imaging due to their capability of ultrasonic transduction in a miniaturized footprint, easily amenable to create large arrays. However, their application space still remains quite open. PMUTs are well fitted to applications in liquid media, such as implantable and underwater devices, due to their inherent acoustic matching and wide bandwidth. Thus, in the first part of this dissertation, novel applications are explored such as PMUT-based intra-body networking, power transfer, source localization, wide-band matching and duplexing.Aluminum Nitride (AlN) has been the piezoelectric material of choice for PMUts in a good part of the work due to its biocompatibility and possibility of single-chip integration with supporting CMOS circuitry. However, Scandium doping of AlN thin films has recently been demonstrated to increase piezoelectric coupling coefficients while introducing ferroelectric properties in the material. However, a simultaneous use of both capabilities has not been demonstrated in the state-of-the-art. The ability of having distinct ferroelectric states, that alter the mechanical performance of the devices, allows for Processing In-Sensor (PIS) with concurrent actuation features and provides the building blocks for analog computing and neuromorphic signal processing capabilities. The second part of the dissertation thus explores the AlScN material integration into novel Ferroelectric Micromachined Ultrasonic Transducers (FMUTs) and their emerging application space.