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

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
$\boldsymbol{2\mu} \mathbf{m}$
- thick PZT film on a
$\boldsymbol{4\mu}\mathbf{m}$
-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
$(\mathbf{e}_{\boldsymbol{31,}\mathbf{f}}\boldsymbol{\sim 16}\mathbf{C}/\mathbf{m}^{\boldsymbol{2}})$
and compliance of the diaphragm designed for ranging applications, we demonstrate large tunability in frequency (120-210kHz),
$\mathbf{K}_{\mathbf{t}}{}^{\boldsymbol{2}}$
(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.