Single-Layer Corrugated Aluminum Nitride Membranes as Piezoelectric Micromachined Ultrasonic Transducers with Differential Readout and Actuation

In this paper we present a novel cost efficient differential PMUT concept based on a single layer corrugated AlN membrane without carrier layer. Each membrane of the realized 2x2 rectangular PMUT array has a radius of 165 μm with a 500 nm AlN layer and a 400 nm Si
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passivation layer. The 2.8 μm tall corrugations create a geometrical separation between peak and valley electrodes, hence regions of tensile and compressive stress, for differential operation with a single piezoelectric layer and no carrier membrane. Furthermore, the corrugations allow for a partial relaxation of the intrinsic material prestresses, mitigating their impact on the device characteristics such as resonant frequency and compliance. The membranes of the array show a resonance at ~338.5 kHz and a peak displacement of up to ~740 nm/V. The array has a strong directivity and generates ~117 dB SPL at 1 cm on-axis distance at an actuation voltage of 4 V
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