Guided Acoustics and Inverse Problem

Person in charge : Farouk BENMEDDOUR

The research activities developed within this theme aim at the understanding of the interaction phenomena between guided acoustic waves and matter. These activities require the theoretical and experimental study of the excitation and propagation of these waves in different media. One of the objectives is the development of characterization methods adapted and optimized for the non-destructive evaluation of physical and structural properties of materials. The main activities concern the Laser Ultrasound (LU), interdigital transducers (IDT),...

The terminology "Laser Ultrasound" refers to a method for non-contact generation and detection of acoustic waves. Compared to other routes, this solution therefore does not require the use of a coupling medium. It also allows the characterization of materials brought to high temperatures and is easily adapted to structures with complex geometries for which the implementation of conventional methods is often difficult.

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Ultrasonic wave excitation is achieved by irradiating the sample with a focused laser pulse of well-defined spatial and temporal shapes. This results in a local absorption of the light radiation generating a sudden heating of the impacted area. Depending on the incident light power density, two main modes of photoelastic generation can be distinguished: the thermoelastic mode and the ablation mode.

In addition, the detection of ultrasonic waves is also performed optically by interferometry. The principle of this detection is most often based on the measurement of the normal displacement of the sample surface when the ultrasonic wave passes.

The principle of this detection is based on the measurement of the normal displacement of the sample surface when the ultrasonic wave passes.

The principle of this detection is based on the measurement of the normal displacement of the sample surface when the ultrasonic wave passes.

Our activities are currently focused on the modeling of photoelastic sources and the characterization of layer-on-substrate type structures by surface or plate waves. In addition, we are interested in the wave-defect interaction and in the development of techniques to work at higher frequencies.

We are also interested in the development of a system of ultrasonic waves for the characterization of structures.

Figure 1: Modeling the excitation of ultrasonic waves by laser source- Figure 2: Ultrasound-Laser device

Figure 3: Ultrasound-Laser-derived plate waves and associated dispersion curves for an indium layer on silicon

Complementary to the previous activity, we are also developing a method for characterizing gradient layers and materials based on the implementation of interdigital sensors (IDT). This one allows the generation of quasi-monochromatic surface acoustic waves whose frequency is "adaptable" to the considered structure.

Figure 4: Modeling and generation of surface waves by IDT - Comparison of displacements generated by the modeling wave and experimentation

Major results

A first highlight concerns Laser Ultrasound, which is increasingly used for non-destructive testing and evaluation of materials. They are of particular interest to various industries such as electronics and transportation. Indeed, this method of excitation and detection of ultrasonic waves has some advantages compared to more conventional methods often based on the use of piezoelectric transducers. Among these advantages, we can mention the wide bandwidth, the absence of contact, the possibility to study structures of complex geometry.

Thin-film-on-substrate type structures are of paramount importance in the field of microelectronics. The physical properties of these structures strongly depend on the one of the layer that is why it is often essential to be able to determine the elastic parameters of the film as well as its thickness. For this purpose, we have privileged the use of guided acoustic waves (plate and Rayleigh modes) excited in a pulsed regime in a frequency range up to 45 MHz. Various metallic deposits of micrometer thickness on silicon substrate have been characterized. Innovative inversion methods based on neural networks and multimodal analysis have been developed. These methods have been validated with finite element simulations and have allowed to obtain the desired characteristics. Original complementary studies have also highlighted the possibility of obtaining from specific surface patterns effects similar to those of a frequency selective acoustic lens.

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Non-contact Non-Destructive Testing is also of great interest in the transportation field. Thanks to the ECOCND project (ANR ECOTECH program), we have been particularly interested in the characterization of crack-like defects by an original multi-wave analysis  including mode conversion phenomena. Different experiments and simulations have shown that it is possible to optimize the controls performed so far in order to obtain the characteristic parameters of the considered defect [RSI 2009, RSI 2010, MST 2012].

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Figure 5: Plate modes obtained by 2DFFT for a structure consisting of a gold layer deposited on a silicon substrate.

A second highlight concerns the characterization of surface residual stress fields by HF surface acoustic waves generated by IDT-SAW MEMS. The study and control of residual mechanical stresses is becoming increasingly important in many fields such as microelectronics, and gradient materials, in order to meet new functional requirements. The effects of these stresses can be harmful in some applications (breakdown phenomenon) and sometimes beneficial as for example to improve the transport properties in silicon or to mechanically reinforce the surface of materials. It is therefore necessary to control the stress levels and consequently important to have tools for characterizing these stresses adapted to both the scale and the nature of the deposited materials.

We have developed a method for characterizing surface residual stress fields based on the study of HF surface acoustic wave (SAW) propagation. From the theory of acoustoelasticity, the perturbations generated by the presence of stress fields on these surface waves have been analyzed theoretically [APL 2008] and experimentally on amorphous media and for different surface stress fields [JASA 2006]. The design and implementation of specific IDT-SAW MEMS sensors has allowed the generation of quasi-monochromatic Rayleigh-like surface waves over a wide range of frequencies [Patent No. FR2977940 (A1) 2013-01-18; RSI 2011]. The study of dispersion phenomena associated with inverse method procedures led to the estimation of the thicknesses of the stressed cortical zones as well as the values of the residual stresses [APL 2012].

Figure 6: SAW dispersion curves for three surface fields of residual stress

Finally, a third highlight concerns the understanding of the physical phenomena of the interaction of Lamb waves with damage. This work shows that it is possible to understand and quantify the interaction of a Lamb mode in the presence of complex damage only from calculations performed for elementary damage [NDT&E Int, 41, 5 (2008) ; NDT&E Int., 41, 1 (2008)]. These results were further validated experimentally using an original device for generating and receiving only one Lamb mode at a time [Ultrasonics, 49, 2 (2009)].

Work in progress - Prospects

Current work is interested in the adhesion of thin films and coatings (LU and IDT method) and in the characterization of defects that may impact them. Other studies are also carried out in order to better understand, interpret and predict the various phenomena of acoustic wave diffraction on a singularity of the structure. All of this work can lead to applications in microelectronics or in the safety of installations, particularly in the field of transportation. For example, within the framework of a thesis in partnership with the IRT RAILENIUM, some limits of the non-contact control for applications related to the railway transport have been studied.

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Collaborations and theses in progress

  • Collaboration with Tongji University (Shanghai).
  • Hubert Curien Program (Catholic University of Leuven).
  • European Interreg project (INISMA, LMP...).
  • ANR ECOCND project (CEA, Institut de Soudure, Ixtrem, Holo3).
  • CIFRE thesis ( VALLOUREC, SKF, Soben).
  • IRT RAILENIUM thesis