Sputtered PZT pMUT with Bias-Tunable Electromechanical Coupling Coefficient for Air-coupled Ranging Applications

Abstract

We report preliminary wafer-level measurement results of air-coupled phase vapor deposition (PVD) PZT pMUTs for ranging applications recently fabricated in our Lab-in-Fab 8-inch line. Dual-port PVD PZT pMUTs designed to resonate in the 100kHz range were fabricated based on a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{2\mu} \mathbf{m}$</tex> - thick PZT film on a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{4\mu}\mathbf{m}$</tex> -thick silicon diaphragm. The standard deviation of the PZT stack capacitance was <2% across the wafer. Despite the sensitivity of compliant diaphragms to film residual stress, the standard deviation of frequency was <5%. Out of 16 sites across wafer probed, all devices were verified to be in good working condition. Given the strong piezoelectric constant of the PZT film <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{e}_{\boldsymbol{31,}\mathbf{f}}\boldsymbol{\sim 16}\mathbf{C}/\mathbf{m}^{\boldsymbol{2}})$</tex> and compliance of the diaphragm designed for ranging applications, we demonstrate large tunability in frequency (120-210kHz), <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{K}_{\mathbf{t}}{}^{\boldsymbol{2}}$</tex> (1-7.3%), and quality factor (50–100) for a bias voltage range of0-40V. Results have been obtained without poling of the PVD PZT film. No unresponsive devices were found.