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/m
²
. 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/m
²
. A design and micromachining concpet for easy scaling in frequency has been developed. The electromechanical coupling coefficient (k
²
) and the quality factor (Q) of rectangular clamped elements (with former PZT process) were measured as k
²
=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 k
²
=3% in air. The effect of DC bias voltage on frequency and k
²
has been studied.