Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO3 single crystal.

Ultrasonic transducers are essential for high-frequency imaging and non-destructive testing. Prior research has shown that materials like lithium niobate (LiNbO3) offer unique piezoelectric properties suitable for such applications. However, controlling the inversion layer thickness remains a challenge. It was already known that inversion layers influence transducer performance, but no prior work had resolved how to precisely control this layer using temperature. This gap motivated the current study. Researchers sought to develop a method to fabricate high-frequency transducers with controllable inversion layers. The need for precise frequency control in ultrasonic devices remains unmet. Existing methods lack the ability to tailor inversion layer thickness effectively. This paper addresses that limitation by introducing a novel fabrication approach.

The primary aim of this study was to fabricate high-frequency ultrasonic transducers using a half-thickness inversion layer in LiNbO3 single crystal. The specific problem addressed is the lack of precise control over inversion layer thickness in piezoelectric materials. That uncertainty drove the investigation into how annealing temperature affects inversion layer formation. The motivation stems from the need for transducers operating in the 30–60 MHz range for biomedical and industrial applications. Prior methods lacked reproducibility in inversion layer thickness. This study aimed to demonstrate that inversion layer thickness can be controlled through thermal treatment. The goal was to achieve consistent resonant frequencies and broadband performance. The study also aimed to validate analytical models against experimental data.

The transducers were fabricated using a 36-degree rotated Y-cut LiNbO3 thin plate. The active element thickness was set to 115 micrometers to achieve the desired frequency range. The inversion layer was formed after high-temperature annealing of the sample. The thickness of the inversion layer was controlled by varying the annealing temperature. Silver powder and epoxy composite were used as acoustic matching layers. Parylene was also applied as a matching layer to improve performance. A lossy silver epoxy was selected as the backing material to reduce unwanted reflections. Electrical impedance was calculated using an analytical method for various inversion layer ratios.

The transducers achieved a center frequency between 30 and 60 MHz, as designed. The inversion layer thickness was successfully controlled through temperature variation during annealing. The electrical impedance of the transducers was calculated and compared with measured data. The resonant frequency matched the modeled predictions, confirming the design accuracy. A broadband transducer with a center frequency at 60 MHz was successfully fabricated. The even-order higher frequency performance was demonstrated using the half-thickness inversion layer. The use of silver powder/epoxy and parylene improved acoustic matching. The results suggest that the inversion layer thickness can be tailored for specific frequency applications.

The study demonstrated that inversion layer thickness in LiNbO3 can be controlled via annealing temperature. The fabricated transducers achieved the desired center frequency range of 30–60 MHz. The analytical model provided accurate predictions of resonant frequency. The use of silver powder/epoxy and parylene improved acoustic performance. The half-thickness inversion layer enabled broadband operation at 60 MHz. The results suggest that precise temperature control is essential for inversion layer formation. The study supports the feasibility of using LiNbO3 for high-frequency transducers. The findings may guide future work on optimizing inversion layer properties for specific applications.