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

This work describes a previously overlooked source of measurement drift in picosecond acoustics experiments and demonstrates a solution for ensuring long-term measurement reliability. Picosecond acoustics is a well-suited non-destructive characterization technique used for determining film thickness, elastic properties, and interface quality in multilayer systems. We reveal that significant changes in measured signals develop over time in thin-film stacks studied by picosecond sonar. These temporal variations occur rapidly after sample deposition and persist for months, manifesting most prominently in the thermal exponential background, but also affecting echo shapes and Brillouin oscillation phases. Our systematic investigation demonstrates that these measurement drifts originate from aging of the titanium transducer layer when exposed to ambient atmosphere. Such aging effects can compromise data reliability when comparing measurements taken at different times or when analyzing subtle signal features. To avoid this issue, we show that the signal variation can be completely eliminated by passivating the Ti layer with a thin 10 nm Si₃N₄ coating. These results provide guidance for reliable picosecond acoustics characterization and highlight the critical importance of considering transducer stability in ultrafast acoustics.