Duquennoy Marc, Smagin Nikolay, Kadi Tahar, Ouaftouh Mohammadi, Jenot Frédéric
IEMN (UMR CNRS 8520), University Polytechnique Hauts-de-France, CNRS, University Lille, F-59313 Valenciennes, France.
Sensors (Basel). 2022 Oct 1;22(19):7464. doi: 10.3390/s22197464.
In the ultrasonic non-destructive evaluation of thin films, it is essential to have ultrasonic transducers that are able to generate surface acoustic waves (SAW) of suitably high frequencies in a wide frequency range of between ten and several hundred megahertz. If the characterization is carried out with the transducer in contact with the sample, it is also necessary that the transducers provide a high level of mechanical displacement (>100 s pm). This level allows the wave to cross the transducer−sample interface and propagate over the distance of a few millimeters on the sample and be properly detected. In this paper, an emitter transducer formed of interdigitated chirp electrodes deposited on 128° Y-cut LiNbO3 is proposed. It is shown that this solution efficiently enables the generation of SAW (displacement level up to 1 nm) in a frequency range of between 100 and 240 MHz. The electrical characterization and a displacement field analysis of SAW by laser Doppler vibrometry are presented. The transducer’s significant unidirectionality is demonstrated. Finally, the characterization of two titanium thin films deposited on silicon is presented as an example. A meaningful SAW velocity dispersion (~10 m/s) is obtained, which allows for the precise estimation (5% of relative error) of the submicrometer thickness of the layers (20 and 50 nm).
在薄膜的超声无损评估中，至关重要的是要有能够在10至数百兆赫兹的宽频率范围内产生适当高频表面声波（SAW）的超声换能器。如果在换能器与样品接触的情况下进行表征，换能器还必须提供高水平的机械位移（>100 spm）。这个水平能使波穿过换能器 - 样品界面，并在样品上传播几毫米的距离，从而被正确检测到。本文提出了一种由沉积在128° Y切割铌酸锂上的叉指啁啾电极构成的发射换能器。结果表明，该解决方案能够在100至240 MHz的频率范围内有效地产生SAW（位移水平高达1 nm）。文中给出了通过激光多普勒振动测量法对SAW进行的电学表征和位移场分析。证明了该换能器具有显著的单向性。最后，给出了以沉积在硅上的两种钛薄膜为例的表征结果。获得了有意义的SAW速度色散（约10 m/s），这使得能够精确估计（相对误差为5%）层（20和50 nm）的亚微米厚度。
