High Q-factor Ba0.25Sr0.75TiO3 thin film bulk acoustic wave resonators: growth conditions and correlations with microstructure

The electrically tunable thin film bulk acoustic wave resonators (TFBARs), utilizing electric field induced piezoelectric effect in paraelectric phase ferroelectrics, enable development of novel reconfigurable/adaptable circuit architectures. However, the practical applications are hindered by their relatively low acoustic Q-factor limited by extrinsic loss mechanisms associated with structural imperfections. The Qf product of the best reported tunable BaxSr1-xTiO3 (BSTO) TFBARs are in the range 400-600 GHz. In this work correlations between microstructure and Q-factor of tunable solidly mounted BSTO TFBARs are studied using analysis of test structures prepared at different growth temperatures of the BSTO films varying in the range 450-650 C. The BSTO films are deposited by magnetron sputtering in on-axis configuration at 2 mTorr gas pressure. These deposition conditions support formation of the films with low grain misorientation which, in turn, reduces the generation of shear waves leaking through the Bragg reflector. 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, wave scattering by surface roughness and resonance broadening by local thickness variations. The observed drop in the Q-factor with growth temperature above 590 C is associated mainly with sharp increase in the roughness (Fig. 1) at top (Fig. 2) and bottom (Fig. 3) interfaces and corresponding acoustic losses due to wave scattering and resonance broadening by local thickness variations. The dashed line in Fig. 1 is a simulated quality factor taking into account loss associated with scattering by top interface roughness. The increase in the roughness is caused by a deformation of the Pt bottom electrode (Fig. 3) during heating up to the BSTO film growth temperature due to large differences in the thermal expansion coefficients of the Pt and the SiO2 layers of the Bragg reflector below. The TFBAR utilizing BSTO film grown at 585 C reveals series resonance at 5.15 GHz with Q=360 (Qf=1854), tunability of the resonance frequency 1% and electromechanical coupling coefficient 2.5% at 25V dc bias