Flexible PZT-Based Row-Column Addressed 2-D PMUT Array

This article reports a $30\times 12$ row-column (RC) addressed flexible piezoelectric micromachined ultrasound transducer (PMUT) array with a top-down fabrication process. The fabrication uses a temporary carrier wafer from which the array device is released by deep reactive ion etching (DRIE). About 0.8- $\mu $ m-thick sol-gel processed lead zirconate titanate (PZT) thin film acts as the active piezoelectric. The flexible PMUT membrane includes the PZT film and a 14- $\mu $ m polyimide as a passive layer. A sidewall made of polyimide measuring $21~\mu $ m in thickness with a cavity of $100~\mu $ m in diameter is realized by reactive ion etching (RIE). Laser Doppler vibrometer (LDV) characterization of the PMUT indicates 2.7 and 2.1 MHz as the resonance frequency in-air and underwater, respectively. Excitation of a single PMUT element coupled with 5-V direct current (dc) bias results in 1.2-nm/V sensitivity in-air, whereas when the same is excited along with 10-V dc bias, a pressure response of 40 Pa/V at 1 cm is measured underwater using a hydrophone. The presented results under bending to an 8-mm bending radius show the potential for wearable applications in shallow-depth regions subject to further optimization.