Simulation and characterization of piezoelectric micromachined ultrasonic transducers (PMUTS) based on PZT/SOI membranes

Piezoelectric micromachined ultrasonic transducers for airborne and immersed applications working in the frequency range from 20 kHz in liquid to 750 kHz in air have been fabricated and characterized, as well as simulated by finite element modelling. The basic element consisted of a oxidized and platinized silicon membrane coated with a 2 mum thick (100)-textured Pb(Zr,Ti)O-3 (PZT) thin film deposited by sol-gel techniques. SOI wafers have been applied to obtain a good definition of the silicon part of the membrane. The unclamping of the silicon membrane at the border increases drastically the coupling factor. For unclamped structures the membranes completely covered with top electrode show the highest coupling coefficient (k(2) = 5.6%). Immersed (in Fluorinert(TM)) membranes show 7 times smaller quality factor, while the coupling factor k(2) remains the same. The obtained structures were enough sensitive to detect acoustic waves in air and in liquid emitted from the same type of elements at a distance of 20 and 2 cm, respectively. Good agreement between the experimental results (membrane deflection, admittance in air and in liquid) and FEA simulations has been obtained.