Guided SH-SAW characterization of elasticity variations of mesoporous TiO2 sensitive films during humidity sorption

Anomalous responses of acoustic humidity sensors coated with mesoporous titania have been observed at high humidity levels, due to capillary contraction that alters the film mechanical behavior. As the methods commonly used to assess the elasticity of thin films are difficult to apply during sorption, a dedicated method for the characterization of the variations of the elastic shear modulus of thin films under humidity exposure has been developed. The method combines a guided shear-horizontal surface acoustic wave (guided SH-SAW or Love wave) platform with environmental ellipsometric porosimetry (EEP). In the presented approach, EEP measures the thickness of the film and its adsorbed humidity volume fraction under vapor exposure, while the guided SH-SAW platform provides the phase velocity shifts induced by water sorption. These parameters then feed an accurate model of Love wave propagation in the multilayered platform for the derivation of the shear modulus of the sensitive film. The method was applied to 100±10 nm thick mesoporous titania films, with 25±1% porosity, under relative humidity exposure in the 3%-95% range. It successfully determined decreases of 39% and 67% of the shear modulus during adsorption and desorption, respectively, from a 3.1 GPa initial value.