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

Area of Science:

• Materials Science
• Electrical Engineering
• Acoustics

Background:

• Piezoelectrical micromachined ultrasound transducers (PMUTs) are crucial for various applications.
• Optimizing PMUTs integrated with CMOS technology is essential for enhanced performance.
• Residual stress in thin layers can cause membrane buckling, potentially degrading performance.

Purpose of the Study:

• To develop guidelines for optimizing monolithically integrated PMUTs on CMOS technology.
• To investigate the impact of thin AlN piezoelectric layers and Si3N4 passive layers on acoustic pressure.
• To explore the effect of residual stress-induced buckling on PMUT performance.

Main Methods:

• Finite Element Modeling (FEM) simulations were employed to study PMUT behavior.
• Experimental characterization was conducted on fabricated PMUTs.
• Optimization strategies focused on electrode placement to leverage membrane buckling.

Main Results:

• PMUTs with thin AlN and Si3N4 layers were simulated and characterized.
• It was demonstrated that membrane buckling, due to residual stress, can be beneficial for PMUT performance.
• Fabricated PMUTs showed enhanced transmitter (5 kPa V⁻¹) and receiver (12.5 V MPa⁻¹) capabilities.

Conclusions:

• Guidelines for optimizing CMOS-integrated PMUTs were established.
• Strategic electrode placement can harness residual stress-induced buckling for improved PMUT efficiency.
• The developed PMUTs outperform previous devices on the same technology and state-of-the-art alternatives.