In recent decades, micromachined ultrasonic transducers (MUTs) have been investigated as an alternative to conventional piezocomposite ultrasonic transducers, primarily due to the advantages that microelectromechanical systems provide. Miniaturized ultrasonic systems require ultrasonic transducers integrated with complementary metal-oxide-semiconductor circuits. Hence, piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs) have been developed as the most favorable solutions. This paper reviews the basic equations to understand the characteristics of thin-film-based piezoelectric devices and presents recent research on pMUTs, including current approaches and limitations. Methods to improve the coupling coefficient of pMUTs are also investigated, such as device structure, materials, and fabrication techniques. The device structure improvements include multielectrode pMUTs, partially clamped boundary conditions, and 3D pMUTs (curved and domed types), where the latter can provide an electromechanical coupling coefficient of up to 45%. The piezoelectric coefficient (e31) can be increased by controlling the crystal texture (seed layer of γ-Al2O3), using single-crystal (PMN-PT) materials, or control of residual stresses (using SiO2 layer). Arrays of pMUTs can be implemented for various applications including intravascular ultrasound, fingerprint sensors, rangefinders in air, and wireless power supply systems. pMUTs are expected to be an ideal solution for applications such as mobile biometric security (fingerprint sensors) and rangefinders due to their superior power efficiency and compact size.

This study presents the fabrication and characterization of a piezoelectric micromachined ultrasonic transducer (pMUT; radius: 40 µm) using a patterned aluminum nitride (AlN) thin film as the active piezoelectric material. A 20 × 20 array of pMUTs using a 1 µm thick AlN thin film was designed and fabricated on a 2 × 2 mm2 footprint for a high fill factor. Based on the electrical impedance and phase of the pMUT array, the electromechanical coefficient was ~1.7% at the average resonant frequency of 2.82 MHz in air. Dynamic displacement of the pMUT surface was characterized by scanning laser Doppler vibrometry. The pressure output while immersed in water was 19.79 kPa when calculated based on the peak displacement at the resonant frequency. The proposed AlN pMUT array has potential applications in biomedical sensing for healthcare, medical imaging, and biometrics.

No abstract

No abstract

No abstract