Fabrication and Characterization of AlN-based, CMOS compatible Piezo-MEMS Devices

TL;DR

This paper reports the development of high-quality AlN thin films for piezoelectric MEMS devices, demonstrating controlled growth parameters, stress management, and successful fabrication of PMUTs with promising vibrational properties.

Contribution

It introduces a comprehensive process for fabricating high-quality, CMOS-compatible AlN piezoelectric films with controlled stress and orientation for MEMS applications.

Findings

Achieved <5deg FWHM X-ray diffraction peak width indicating high film orientation

Controlled residual stress from -1.2 GPa to 230 MPa through process parameters

Fabricated PMUTs showing promising vibrational behavior

Abstract

This paper details the development of high-quality, c-axis oriented AlN thin films up to 2 {\mu}m thick, using sputtering on platinum-coated SOI substrates for use in piezoelectric MEMS. Our comprehensive studies illustrate how important growth parameters such as the base Pt electrode quality, deposition temperature, power, and pressure, can influence film quality. With careful adjustment of these parameters, we managed to manipulate residual stresses (from compressive -1.2 GPa to tensile 230 MPa), and attain a high level of orientation in the AlN thin films, evidenced by < 5deg FWHM X-Ray diffraction peak widths. We also report on film surface quality regarding roughness, as assessed by atomic force microscopy, and grain size, as determined through scanning electron microscopy. Having attained the desired film quality, we proceeded to a fabrication process to create piezoelectric micromachined ultrasound transducers (PMUTs) with the AlN on SOI material stack, using deep reactive ion etching (DRIE). Initial evaluations of the vibrational behavior of the created devices, as observed through Laser Doppler Vibrometry, hint at the potential of these optimized AlN thin films for MEMS transducer development.