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This paper 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 μm) and finer pitch than previous processes requiring through-wafer etching. PMUTs were fabricated with diameters from 25 to 50 μm, resulting in center frequencies from 13 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-μm 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 k 2 t = 12.5% and 50-Ω 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 9 × 9 array of 40-μm 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-μm diameter, focused beam over a depth of focus >0.2 mm and increased the pressure to 450 kPa with 18 Vpp input.
Lu et al. (Mon,) studied this question.