Correlations between microstructure and Q-factor of tunable thin film bulk acoustic wave resonators

Correlations between microstructure and Q-factor of tunable solidly mounted Ba_0.25Sr_0.75TiO₃ (BSTO) thin film bulk acoustic wave resonators are studied using analysis of test structures prepared at different growth temperatures of the BSTO films varying in the range 450-650 °C. The observed changes in the Q-factor with growth temperature are correlated with related changes in microstructure, including the grain size, texture misalignment, interfacial amorphous layer, surface roughness, and deterioration of the Bragg reflector layers. The correlations are established through analysis of corresponding extrinsic acoustic loss mechanisms, including Rayleigh scattering at localized defects, acoustic attenuation by amorphous layer, generation of the shear waves leaking into the substrate, waves scattering by surface roughness, and resonance broadening by local thickness variations. It is shown that the waves scattering by surface roughness at the BSTO film interfaces is the main loss mechanism limiting the Q-factor of the BSTO thin film bulk acoustic wave resonators.