A Novel Nondestructive Testing Probe Using AlN-Based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs)

Abstract

Ultrasonic nondestructive testing (NDT) usually utilizes conventional bulk piezoelectric transducers as transceivers. However, the complicated preparation and assembly process of bulk piezoelectric ceramics limits the development of NDT probes toward miniaturization and high frequency. In this paper, a 4.4 mm × 4.4 mm aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) array is designed, fabricated, characterized, and packaged for ultrasonic pulse-echo NDT of solids for the first time. The PMUT array is prepared based on the cavity silicon-on-insulator (CSOI) process and packaged using polyurethane (PU) material with acoustic properties similar to water. The fabricated PMUT array resonates at 2.183 MHz in air and at around 1.25 MHz after PU encapsulation. The bandwidth of the packaged PMUT receiver (244 kHz) is wider than that of a bulk piezoelectric transducer (179 kHz), which is good for axis resolution improvement. In this work, a hybrid ultrasonic NDT probe is designed using two packaged PMUT receivers and one 1.25 MHz bulk transmitter. The bulk transmitter radiates an ultrasonic wave into the sample, and the defect echo is received by two PMUT receivers. The 2D position of the defect could be figured out by time-of-flight (TOF) difference, and a 30 mm × 65 mm detection area is acquired. This work demonstrates the feasibility of applying AlN PMUTs to ultrasonic NDT of solids and paves the way toward a miniaturized NDT probe using AlN PMUT technology.

Keywords: bandwidth; piezoelectric micromachined ultrasonic transducers; time of flight; ultrasonic nondestructive testing.

Figure 1

(a) Structure of the PMUT. (b) Cross-sectional view of the FEM simulation model. (c) Simulation results of first-order resonant frequency in air and water with different radiuses.

Figure 2

(a–f) Fabrication process flow of the PMUT array. (g) Optical image of the fabricated PMUT array.

Figure 3

Electromechanical characterization of the PMUT array. (a) Measured electrical impedance in air. (b) Measured mechanical vibration performance in air. (c) Measured mechanical vibration performance in deionized water.

Figure 4

(a) Structure of the hybrid NDT probe. (b) Optical image of the NDT probe. (c) Schematic of 2D defect localization.

Figure 5

Propagation simulation. (a) Schematic of the first simulation model. (b) Pressure distribution in solid sample. (c) Schematic of the second simulation model. (d) Sound pressure distribution in matching layer. (e) Excitation and defect echo from different depths.

Figure 6

Setup of the ultrasonic testing experiment.

Figure 7

Defect echo received by PMUT sensor: (a) 50 mm deep defect echo, (b) 50 mm deep and 5 mm horizontal shift defect echo, (c) 73 mm deep and 10 mm horizontal shift defect echo, (d) 97 mm deep and 15 mm horizontal shift defect echo.

Figure 8

(a) Schematic of the receiving experiment. (b) Excitation signal. (c) Received signal by bulk transducer and MEMS sensor. (d) FFT of the received signal.