Microfabrication and characterization of a new box-shaped high frequency (7.5 MHz) low aperture 2D phased ultrasound transducer array

Abstract

Mini electronics in wearables inspired this study. Thus, this work offers a new box-shaped high frequency (7.5 MHz) low aperture 2D phased sparse array ultrasonic transducer developed, built, and characterized. The capacity of matrix or 2D phased arrays to generate ultrasound beams without requiring any form of motion or mechanical steering holds potential value in the biomedical sonographic domain. However, these systems need a large number of piezoelectric elements to sample the active aperture, which is smaller than λ/2 wavelength between them, necessitating the need for a sizable or large transducer. To the best of knowledge, this is the first endeavor to design and microfabricate a 7.5 MHz transducer array, based on commercial PZT-5H polycrystalline materials, as tiny as 70x70 µm per transducer with a pitch of 102 µm to maintain an inter-element separation below 50% of the lambda. The study employs a square box-shaped structure that houses the transmitters and receivers perpendicular to each other, resulting in a reduced aperture and compact design compared to different commercial designs. This transducer not only provides satisfactory longitudinal k33 coefficient (0.45-0.5), acoustic pressure (2.1 kPa), sound pressure level (180 dB), low Q-factor (1.19), thermal stability, and high bandwidth (5.6 MHz, 73.41%), while minimizing cross-talk (<-50 dB), but also reduces the overall transducer area due to its unique sparse array configuration, resulting in a diminutive size (3.3 mm x 3.3 mm).