Measurement of the attenuation of elastic waves at GHz frequencies using resonant thickness modes

Abstract

We present an evaluation of attenuation of elastic waves in the GHz range, based on the decay of thickness-stretch resonances of plates. We measured the elastic response of micro-scale aluminum plates, using a laser-ultrasound technique. The thermo-elastic laser excitation provides significant coupling into thickness-stretch modes in the frequency range above 1.5 GHz. To suppress interference from other resonant and transient modes, we used an excitation spot size well above the plate thickness and applied signal processing in the time and frequency domain. We adapted existing theory on the decay of thickness-shear modes to apply for thickness-stretch modes, validated the derived theory with simulations, and applied it to experiments. A set of samples with different thicknesses in the range of 1.1–2.15 μm allowed us to obtain damping values in the corresponding frequency range of 1.5–3 GHz.

Analytical model of the acoustic response of nanogranular films adhering on a substrate

“…Photoacoustics investigations of the bulk properties of non-periodic nanogranular films have also been performed in several contexts over granularities ranging from few nm [4,29,30], to hundreds of nm [31,32] up to the micron scale [33]. As for interface properties, photoacoustic investigations mainly focused on homogeneous thin films [34][35][36][37][38][39][40][41][42][43][44], nanogranular thin film interfaces remaining relatively unexplored. The difficulty is to address 'patched' interfaces as the one emerging between an aperiodic granular film and the adhering substrate, disorder being the critical aspect [29].…”