Development of High Frequency Interdigital Transducers for NDT of Thin Films

International audience ; Various engineering systems components suffer destructive effects on their surfaces, like wear, fatigue, or corrosion during their operation. Protecting the surface by hardening treatments or applying coatings is a widespread way to extend their life and improve the product quality. Important properties of surface films and coatings are thickness, adhesion to the substrate, hardness, elastic modulus, thermal conductivity, electrical conductivity and others. Surface acoustic waves are elastic vibrations, which propagate along the surface of the material. They are very sensitive to films properties and surface treatments, since the wave energy is concentrated near the surface. Therefore, there is a significant interest in using this acoustic wave mode for nondestructive testing. Whereas the wave velocity is constant for homogenous materials, the phase velocity c depends on frequency f for coated and surface-modified materials. This phenomenon, termed dispersion, can be used to determine mechanical and geometrical film parameters such as Young?s modulus and film thickness. Especially, Young?s modulus is an important parameter for nondestructive characterization of film materials. In order to determine the parameters of the film material, the dispersion curve c (f) is measured and fitted by a theoretical curve. The sensing depth of the wave can be varied by varying the frequency of the wave. The penetration depth is lower for higher the frequencies due to the relation λ=c⁄f. Films with thickness of few nanometers on substrates with low ultrasonic damping such as silicon single crystals can be investigated, using high-frequency waves (f > 200 MHz). In order to generate these high frequency surface waves, we have developed interdigital transducers. In order to obtain a wide frequency range Rayleigh waves generation, the chirped-type, interdigital transducers (IDT) have been developed. These transducers have been optimized, characterized and used in several NDT applications.