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

Abstract

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.

Piezoelectric Micromachined Ultrasonic Transducers (PMUTs): Performance Metrics, Advancements, and Applications

“…High temperatures are required for thin film deposition of PZT through sol-gel, chemical deposition, and sputtering. High-temperature thin film PZT deposition imposes difficulties on MEMS integration with CMOS due to temperature limitations during fabrication for the CMOS process [ 17 , 35 ]. Another limitation of PZT-based PMUTs is the release of harmful lead into the atmosphere during high-temperature deposition [ 36 , 37 ].…”

“…Higher acoustic pressure output is required to achieve larger imaging depth and detection ranges for medical imaging and range-finding applications. The acoustic pressure output can be related to the axial pressure amplitude P ( z ) at a distance z from the transducer, given by Equation (13) [ 35 , 64 ]. …”

“…AlN is a viable choice due to its low temperature thin film deposition compatible with the CMOS process, which enables monolithic integration of MEMS. An AlN-based PMUT on CMOS has been reported to implement a fully functional transceiver system with better performance and low package cost [ 35 , 37 , 73 ].…”

Single-cell system using monolithic PMUTs-on-CMOS to monitor fluid hydrodynamic properties

“…Moreover, the presented system is monolithically integrated over a preprocessed CMOS substrate with the adequate circuitry for PMUT driving and sensing. Different from our previous works 24 , 25 , in this paper AlN doped with Sc is used as the piezoelectric material, providing benefits in terms of piezoelectric transduction coefficients 26 . This single-cell or lab-on-chip for liquid characterization could be easily integrated in a microfluidic cell or hand-held devices of small size, which will make it competitive with respect to other systems 2 , 21 , 22 .…”