High frequency SAW-IDT MEMS sensors for ultrasonic characterization of thin films deposited on substrates

The use of coatings and thin films deposited on substrates is highly sought-after in many applications. The objectives of these coatings and deposits are numerous. It may be used, for example, to improve the durability of structures, including their resistance to wear and fatigue or to obtain specific physical or electronic properties. The characterization of these coatings and these layers to determine their properties (thickness, elastic constants, etc.) is key in monitoring the condition of the parts and ensuring optimum operation when in use. Various ultrasonic methods can be used for the non-destructive testing of these materials but the surface acoustic wave (SAW) are particularly interesting because the energy is concentrated within a layer under the surface of about one wavelength thick. It thus constitutes an interesting candidate for the characterization of plate or layer on substrate-type structures. To test the materials at different depths, different excitation frequencies are used to obtain the required wavelengths. Traditionally, it is possible to use wedge sensors to generate SAW. However, beyond 10MHz, the losses and attenuations related to this sensor technology become too significant. In order to characterize thin coatings, it is necessary to select higher frequencies. Therefore, in this study, the idea was to use SAW-IDT MEMS Sensor to generate HF surface waves between 5 and 50MHz to characterize thin films.