Piezoelectric micromachined ultrasonic transducers based on PZT thin films

This paper describes fabrication arid characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-/spl mu/m-thick Pb(Zr/sub 0.53/Ti/sub 0.47/O/sub 3/) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially undamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-/spl mu/m plate and amounted to k/sup 2/=5.3%. This value compares well with the calculated value based on measured small signal dielectric (/spl epsi/=1050) and piezoelectric (/spl epsi//sub 31,f/=15 Cm/sup -2/) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasiidentical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid.