Development of a Broadband (100–240 MHz) Surface Acoustic Wave Emitter Devoted to the Non-Destructive Characterization of Sub-Micrometric Thin Films

Abstract

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).

Simultaneous Determination of Young's Modulus and Density of Ultrathin Low‐k Films Using Surface Acoustic Waves

“…[27,28] Duquennoy et al used a broadband 100-240 MHz SAW-IDT to measure the mechanical properties of metal films. [29,30] DOI: 10.1002/pssa.202300860…”

“…[ 27,28 ] Duquennoy et al used a broadband 100–240 MHz SAW‐IDT to measure the mechanical properties of metal films. [ 29,30 ] Despite considerable progress, the majority of studies have focused on single‐parameter characterization, which ignores the potential of SAW technique for multi‐parameter determination.…”

Nondestructive Determination of Young's Modulus of Interconnecting Thin Films by a Broadband Surface Acoustic Wave‐Interdigital Transducer

“…[ 23 ] The most recent work by Duquennoy et al investigated the sensitivity of SAW propagation to ultrathin chromium films tens of nanometers thick, and the Young's modulus was inverted by the SAW dispersion curve. [ 24 ] Moreover, a broadband 100–240 MHz SAW‐IDT, consisting of interdigitated chirp electrodes, was used to analyze the displacement field of the SAW and accurately estimate the thickness of the submicrometer titanium layer. [ 25 ] For all the studies mentioned earlier, SAW signals were collected using laser Doppler vibrometry.…”

Characterization of a Piezoelectric Acoustic Sensor Fabricated for Low-Frequency Applications: A Comparative Study of Three Methods

“…Ultrasound measurement systems have gained significant popularity over the last few decades, and their applications span across several industries, including non-destructive testing (NDT) for predictive maintenance and fault detection [1][2][3][4][5][6][7] medical acoustics for diagnosis and ultrasound scans [8][9][10], and communication and monitoring of marine environments [11][12][13][14][15][16]. Piezoelectric transducers are the primary means of generating acoustic energy in most of these systems.…”