Characterization of AlN thin film piezoelectric coefficient d31 using Piezoelectric Micromachined Ultrasonic Transducers (PMUTs)

Abstract

Abstract The transverse piezoelectric coefficients d 31 of piezoelectric membranes is fundamental to the design, simulation, optimization, and applications of piezoelectric devices using Aluminum Nitride (AlN). In this work, a method based on the inverse piezoelectric effect is proposed to characterize d 31. Based on the piezoelectric equation and the thin film vibration equation, the central displacement of the Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is linearly related to the DC voltages when the PMUTs are in the linear operating region. The central displacement of PMUTs is measured by 3D Measuring Laser Microscope at various DC voltages. The finite element method (FEM) is combined with change of d 31 to approximate the relationship between displacement and voltage. Then, The proposed method is verified for its accuracy through the use of the cantilever technique where the cantilever is fabricated on the same wafer as the above PMUTs. The d 31 of the AlN thin film obtained by the proposed method and the cantilever method are -1.58pC/N and -1.60pC/N. The error of the two methods is within 2%. Ultimately, the extraction and measurement of d 31 can be achieved by the proposed method.