Enhancing AlN PMUTs’ Acoustic Responsivity within a MEMS-on-CMOS Process

In this paper, guidelines for the optimization of piezoelectrical micromachined ultrasound transducers (PMUTs) monolithically integrated over a CMOS technology are developed. Higher acoustic pressure is produced by PMUTs with a thin layer of AlN piezoelectrical material and Si3N4 as a passive layer, as is studied here with finite element modeling (FEM) simulations and experimental characterization. Due to the thin layers used, parameters such as residual stress become relevant as they produce a buckled structure. It has been reported that the buckling of the membrane due to residual stress, in general, reduces the coupling factor and consequently degrades the efficiency of the acoustic pressure production. In this paper, we show that this buckling can be beneficial and that the fabricated PMUTs exhibit enhanced performance depending on the placement of the electrodes. This behavior was demonstrated experimentally and through FEM. The acoustic characterization of the fabricated PMUTs shows the enhancement of the PMUT performance as a transmitter (with 5 kPa V−1 surface pressure for a single PMUT) and as a receiver (12.5 V MPa−1) in comparison with previously reported devices using the same MEMS-on-CMOS technology as well as state-of-the-art devices.

Acoustic Characterization Of Sudan Black Nanobubbles For Multimodal Ultrasound & Photoacoustic Imaging

Dyed, shelled, nanobubbles (NBs) have recently been proposed as contrast agents for multimodal ultrasound-photoacoustic (US-PA) imaging. However, changes to the shell composition due to the presence of the dye can modify the response of bubbles to ultrasound. In this work, the effects of the dye, Sudan Black B, on a formulation of lipid shelled NBs are studied. Formulations were produced with increasing concentration of Sudan Black B. The size and concentration of activated NBs were tested. The surface tension of bulk lipid solution was measured using pendant drop tensiometry and activated bubble solutions were measured for single bubble and bubble population response to incident ultrasound. While results show no statistically significant effect of Sudan Black concentration on bubble concentration or size, surface tension increased linearly with dye concentration to a maximum increase of 13%. With the addition of Sudan Black B, single bubble experiments show an increase in the contribution of bubble growth signals, a decrease in contribution of nonlinear signals, and a decrease in bubble destruction. The results presented in this work indicate that the presence of Sudan Black B in the lipid shell of a nanobubble may increase the shell permeability impacting stability of the bubble population and their potential for multimodal US-PA imaging.