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

TL;DR

This study investigates how aging affects thin-film multilayers in picosecond acoustics experiments, revealing that environmental exposure causes significant signal changes that can be mitigated by passivation.

Contribution

It demonstrates the impact of aging on picosecond acoustic signals in thin films and introduces a passivation method to prevent these effects, ensuring measurement stability.

Findings

Aging causes changes in signal shape and phase in thin films.

Titanium transducer layers undergo oxidation and gas adsorption over time.

Passivation with Si3N4 prevents aging-related signal alterations.

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.