Modeling, Fabrication and Characterization of Piezoelectric Micromachined Ultrasonic Transducer Arrays

This thesis presents high fill-factor piezoelectric micromachined ultrasonic transducer (PMUT) arrays fabricated via a novel process using cavity SOI wafers. The simple three-mask fabrication process enables smaller diameter PMUTs (25 microm) and finer pitch than previous processes requiring through-wafer etching. PMUTs were fabricated with diameters from 25 microm to 50 microm, resulting in center frequencies from 13 MHz to 55 MHz in air. Two types of devices, having different piezoelectric layers, lead zirconium titanate (PZT) and aluminum nitride (AlN), were fabricated and characterized. Comparing 50 microm diameter devices, the PZT PMUTs show large dynamic displacement sensitivity of 316 nm/V at 11 MHz in air, which is ~20x higher than that of the AlN PMUTs. Electrical impedance measurements of the PZT PMUTs show high electromechanical coupling kt2 = 12.5%, and 50 O electrical impedance that is well-matched to typical interface circuits. Immersion tests were conducted on PZT PMUT arrays. The fluid-immersed acoustic pressure generated by an unfocused 9x9 array of 40 microm diameter, 10 MHz PZT PMUTs, measured with a needle hydrophone 1.2 mm away from the array, was 58 kPa with a 25 Vpp input. Beam-forming based on electronic phase control produced a narrow, 150 microm diameter, focused beam over a depth of focus > 0.2 mm and increased the pressure to 450 kPa with 18 Vpp input. Finally, we also demonstrated short-range (~mm) and high-resolution (<100 microm) imaging based on piezoelectric micromachined ultrasonic transducers (PMUTs) and a 1.8 V interface ASIC. The PMUTs use piezoelectric Aluminum Nitride (AlN), which has the advantages of low-temperature (<400 °C) deposition and compatibility with CMOS fabrication but has a relatively low piezoelectric constant (e31=-0.5 C/m2), making detection of ultrasound signals from tiny (50 microm) PMUTs a challenging task. To solve this problem, we developed an ASIC with a low-noise analog front-end pre-amplifier that is impedance matched to the PMUT. Furthermore, a novel beam-forming and scanning method was demonstrated to achieve a sub-100microm focus size and 70 microm scanning step. Pressure map measurement from phased PMUT array and pulse echo imaging results were demonstrated using 1-D and 2-D phantoms.