Implementing (K,Na)NbO3-based lead-free ferroelectric films to piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers (pMUTs) have attracted widespread attentions on account of their vital applications in medical imaging, fingerprint identification, range-finding, gesture recognition, and so on . The piezoelectric layer, which is the key component for pMUTs, has been dominated by ferroelectric Pb(Zr,Ti)O 3 (PZT) because of its outstanding piezoelectric performance, especially in the case of pMUT transmitters. However, the toxicity of lead restricts their medical-related applications, so developing lead-free pMUTs should be of great significance but remains challenging. As a precedent, the present work pioneered the processing of lead-free (K,Na)NbO 3 (KNN)-based pMUTs, and obtained an ultrahigh transmitting sensitivity of 1250 nm V -1 at 66.2 kHz, which is superior to the majority of PZT-based pMUTs reported so far. The excellent displacement sensitivity can be ascribed to the dense and homogeneous feature, (100)-textured structure, orthorhombic-tetragonal phase coexistence, nanosized domains, great electrical insulation and high piezoresponse of the KNN-based film as well as the the elaborate micromachining process. This work initiated a successful precedent for applying lead-free ferroelectric films to pMUTs also as a breakthrough step in promoting KNN-based films toward practical applications. • Lead-free ferroelectric film has been first integrated into piezoelectric micromachined ultrasonic transducers. • An ultrahigh transmitting sensitivity up to 1250 nm V -1 can be achieved in KNN-based pMUTs, being comparable to Pb-based ones. • The high-performance origin of the KNN-based pMUTs is discussed from the aspects of film properties and device designs.