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Ultrasonic characterization of mesoporous thin films

Thin films are now used in several fields, such as coating, sensors, material tuning. Yet, the determination of mechanical parameters of these films is still a challenge and most of the characterization methods are electromagnetic or electric. In previous works, it has been shown that the transmission spectrum is very sensitive to etching. In this study, investigation of the feasibility of the characterization of thin film deposited on silicon substrate is proposed. The theoretical transmission spectrum is calculated using an unidimensional model. This model is based on normal incidence of the ultrasonic beam. The porosity of the thin film is taken into account using a homogenization model based on Biots theory. Measurements are performed with high frequency transducers (from 20 to 50 MHz) using the insertion-substitution method. The optimization between theoretical and experimental spectra is performed using a genetic algorithm in order to avoid premature convergence on a local solution. Measurements have been performed on 300 nm thick TiO2:SiO2 film which has been deposited using a sol gel method on 500 μm thick silicon substrate. Transmission spectra of the reference (without sample) and with the sample are plotted in the figure below. Another reference is made using the transmission spectrum through the substrate only to characterize more precisely the substrate. A shift of the resonance frequency can be observed, due to the thickness variation induced by the thin film.