Piezoelectric Micromachined Ultrasonic Transducers based on PZT films

Piezoelectric micro machined ultrasonic transducers (pMUT) are piezoelectric laminated plates operating at flexural modes. The pMUT´s fabricated in this work contained a 4 mum thick lead zirconate titanate (PZT) thin film deposited by a sol-gel technique, and exhibiting a permittivity of epsiv_r=1200, and an effective transverse piezoelectric coefficient e_31.f of 12 C/m². A further optimization of the sol-gel process yielded larger grain diameters and consequently improved properties: epsiv_r=1600, e_31.f of 16 C/m². A design and micromachining concpet for easy scaling in frequency has been developed. The electromechanical coupling coefficient (k²) and the quality factor (Q) of rectangular clamped elements (with former PZT process) were measured as k²=4.4% and Q=145 in air for a low frequency transducer (@240 kHz). The 16.9 MHz transducer yielded values of Q=25 and k² =3% in air. The effect of DC bias voltage on frequency and k² has been studied.