Spring 2014 IAB & Research Review
March 5 - 6
High Frequency Piezoelectric Micromachined Ultrasonic Transducer (PMUT) Array
We will present a high frequency and high fill factor piezoelectric micromachined ultrasonic transducer (PMUT) array for the application of fingerprint sensors. It is fabricated for the first time 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. PZT PMUTs with 50 µm diameter show a large dynamic displacement sensitivity of 316 nm/V at 11 MHz, a high coupling ratio kt2 = 8.8%, and 50 ohm electrical impedance that is well-matched to circuits. Resonant frequencies and static displacement of PMUTs with different diameters are measured and agree well with FEM results. 50µm diameter PZT PMUTs have 2.75 nm/V static displacement sensitivity, ~28× higher than that of AlN PMUTs of the same diameter. Acoustic pressure generated by a 9×9 array of 40 µm diameter PZT PMUT’s measured with a hydrophone shows high pressure sensitivity of 20 kPa/V. Phased array simulation shows it is feasible to obtain 50 um focus size which means 500 dpi for fingerprint imaging.