The effect of aging in thin films in the picosecond sonar experiment

Abstract

This work examines aging effects in thin-film stacks studied by picosecond acoustics experiments. The method uses a strain wave created by the absorption of a laser pulse in the top metal transducer layer to study the inner structure and properties of multilayers. We show that significant changes in measured signals developed over time, rapidly after sample deposition and continuing for months. Most apparent was the evolution of the thermal exponential background of the signal, but close inspection showed modifications also in the shape of the echoes and the phase of the Brillouin oscillations. We revealed that these changes originate from the titanium transducer layer exposed to the ambient atmosphere. The aging process involves both - irreversible Ti oxidation and reversible gas/water vapor adsorption. Importantly, these aging effects can be eliminated by passivating the Ti layer with a thin 10 nm Si3N4 coating, providing a simple solution for ensuring long-term measurement stability.