FEM-Based Design of Mass-Spring Acoustic Matching Layers for Ultra-High-Frequency Ultrasonic Transducers with...

High-frequency ultrasonic transducers are pivotal for detecting minute defects, offering distinct advantages in terms of non-destructive evaluation, non-invasiveness, and superior spatial resolution. However, achieving effective focusing and efficient acoustic transmission for ultra-high-frequency ultrasonic transducers is a significant challenge. To address this challenge, a mass-spring acoustic matching layer is designed for a transducer based on a half-concave LiNbO₃ piezoelectric element at 100 MHz. The proposed mass-spring stack, comprising a 0.25 μm Au layer and a 2.5 μm Parylene-C layer, operates within a deposition-friendly thickness range ideal for curved substrates, while a conventional quarter-wavelength Parylene-C layer would necessitate a thickness of 6.0 μm at this frequency. The transducer is modeled in COMSOL Multiphysics 6.1, coupling solid mechanics, electrostatics with piezoelectric effects, and pressure acoustics for the water load. Using a fixed concave geometry (curvature radius is 1 mm, which means the focal length is also 1 mm), a frequency sweep from 50 MHz to 150 MHz is conducted to evaluate performance. Analysis of the pressure distribution in the focal plane reveals that the focal length and the -6 dB beamwidth are predominantly governed by aperture diffraction and exhibit minimal variation upon incorporation of a matching layer. The focal length is approximately 1.02 mm within the excitation frequency range of 70 MHz-150 MHz, and the beamwidth decreases markedly with increasing frequency. At 100 MHz, the measured -6 dB beamwidth is approximately 31.6 μm without a matching layer, 32.3 μm with a quarter-wavelength matching layer, and 31.0 μm with a mass-spring matching layer. Crucially, quantitative comparisons reveal that this design yields a systematically higher focal pressure; near the 100 MHz design frequency, the acoustic pressure amplitude achieved with the mass-spring configuration is 1.5 times greater than that obtained using the quarter-wavelength reference. These research results provide a theoretical basis for the application of ultra-high-frequency ultrasound in the detection of tiny defects.