Correlation of Wafer-scale Film Stress Effects on ScAlN pMUT Parameters

We report preliminary wafer-level measurement results of air-coupled
$\mathrm{S}\mathrm{c}_{0.15}\mathrm{A}\mathrm{l}_{0.85}\mathrm{N}$
pMUTs recently fabricated in our Lab-in-Fab 8-inch line. In this work, we show that the
$\mathrm{K}_{\mathrm{t}}{}^{2}$
and mechanical performance of the devices are correlated to the resonant frequency in connection to film stress. At lower film stresses, the electromechanical coupling,
$\mathrm{K}_{\mathrm{t}}{}^{2}$
, can reach typical values of 3%. While ScAlN-based pMUTs of similar levels of doping concentration have been reported previously,
$\mathrm{K}_{\mathrm{t}}{}^{2}$
has been limited to approximately 2% [1]. In this work, the effects of varying tensile film stress have been recorded to show drastic changes to Kt
²
causing variations on the wafer-level from 0.5% to 3% in relation to the induced local stress. The effect on the resonant frequency of the device was also recorded, ranging from 157kHz to 215kHz. These effects were also validated mechanically under an LDV that show displacement transfer variations of 224 to 557nm/V, whereby the transfer is inversely proportional to the amount of tensile stress.