Self-Poled Bismuth Ferrite Thin Film Micromachined for Piezoelectric Ultrasound Transducers

Piezoelectric micromachined ultrasound transducers (pMUTs), especially those using lead-free materials, are crucial next-generation microdevices for precise actuation and sensing, driving advancements in medical, industrial, and environmental applications. Bismuth ferrite (BiFeO₃) is emerging as a promising lead-free piezoelectric material to replace Pb(Zr,Ti)O₃ in pMUTs. Despite its potential, the integration of BiFeO₃ thin films into pMUTs has been hindered by poling issues. Here, a BiFeO₃ heterostructure compositionally downgraded with Gd doping is developed to introduce compressive strain, resulting in strong self-poling. Utilizing a large-area self-poled thin film over an entire 6-inch wafer, a pMUT with a 6 × 6 array at the device level is designed and evaluated. At a resonant frequency of 21 kHz, the dynamic vibration displacement can reach 24.0 nm. At 500 Hz, far below the resonant frequency of 21 kHz, the pMUT also displays sensitive converse piezoelectric response, even at a high temperature of 200 °C. This work represents a significant breakthrough in lead-free BiFeO₃ thin film for practical sensing applications, paving the way for the transformation of macro-transducers into next-generation functional microdevices.