In recent years, high-frequency focused ultrasound has shown great potential in various fields, including the evaluation and testing of electronic devices, characterization of material micro-mechanical properties, high-resolution biomedical imaging, quantitative research in histopathology, and the development of ultrasonic tweezers. Acoustic lens focused ultrasound transducers are currently one of the main choices for high-frequency focused ultrasound transducers.

Zinc oxide (ZnO) is widely used in high-frequency focused ultrasound transducers due to its excellent piezoelectric properties and low high-frequency loss. However, it is challenging for preparing thick zinc oxide films with a thickness exceeding 10 μm and simultaneously possessing high c-axis orientation and low tensile stress.

Researchers from the Institute of Acoustics, Chinese Academy of Sciences (IACAS), improved the structure-zone model (SZM) applicable to pure metal magnetron sputtered zinc oxide. Based on the improved structure-zone model, they optimized the sputtering pressure and Ar:O₂ gas ratio for growing thick films of zinc oxide by magnetron sputtering, preparing thick ZnO films with both low tensile stress and high piezoelectric performance. Based on this study, combined with finite element method simulation analysis results, researchers designed a high-frequency focused ultrasound transducer with a fused quartz acoustic lens, featuring a center frequency of 50 MHz and a -6 dB bandwidth of up to 119%.

The research, published in the international academic journal Journal of Alloys and Compounds, was supported by the guidance of Academician WANG Chenghao from the Chinese Academy of Sciences and grants from the National Natural Science Foundation of China (No. 11874388, No. 12174420), and the Key Research Program of the Chinese Academy of Sciences (No. QYZDY-SSW-JSC007).

Figure.1 XRD patterns of the ZnO films at different sputtering pressures and Ar-to-O2 ratios. (Image by IACAS)

Figure.2 Photograph of the 50 MHz focused ultrasonic transducer. (Image by IACAS)

Figure.3 Time-domain pulse/echo response and normalized frequency spectrum of the fused quartz acoustic lens focused ultrasonic transducer. (Image by IACAS)

Figure.4 Target image obtained using the transducer. (Image by IACAS)

Reference：

Jinming Ti, Junhong Li, Qingqing fan, Wei Ren, Qing Yu, Magnetron sputtering of ZnO thick film for high frequency focused ultrasonic transducer. Journal of Alloys and Compounds, 2022, 933: 167764. DOI: 10.1016/j.jallcom.2022.167764.