HIGH FREQUENCY AND HIGH FILL FACTOR PIEZOELECTRIC MICROMACHINED ULTRASONIC TRANSDUCERS BASED ON CAVITY SOI WAFERS

This paper presents a high frequency and high fill factor piezoelectric micromachined ultrasonic transducer (PMUT) array, which is fabricated with a simple and CMOS compatible process based on commercially available cavity SOI wafers. This 2-mask process eliminates the need for through-wafer etching and enables 10× higher fill factor and thereby higher acoustic performance. PMUTs based on both lead zirconium titanate (PZT) and aluminum nitride (AlN) piezoelectric layers are designed, fabricated and characterized. For the same 50 µm diameter, PZT PMUTs show large dynamic displacement sensitivity of 316 nm/V at 11 MHz in air, which is ~20× higher than that of AlN PMUTs. Electrical impedance measurements of PZT PMUTs show high electromechanical coupling kt 2 = 12.5%, and 50 Ω electrical impedance that is well-matched to circuits. The resonant frequency and static displacement of PMUTs with different diameters are measured and agree well with FEM results. The acoustic pressure generated by an unfocused 9×9 array of 40 µm diameter PZT PMUTs was measured with a needle hydrophone, showing a pressure sensitivity of 2 kPa/V.