Tunable ferroelectric FBARs: An overview
Paper in proceedings, 2015
The presentation will give an overview focusing the main aspects of the tunable ferroelectric thin film bulk acoustic wave resonators (FBARs) concept and development covering the motivation and state of the art, theory, materials, fabrication, design and evaluation of the FBAR performance. It will be shown that the electrically tunable ferroelectric FBARs, utilizing electric field induced piezoelectric effect in ferroelectrics, have been intensively developed for the last years. These intrinsically tunable FBARs are enabling components for emerging novel reconfigurable/adaptable circuit architectures for advanced microwave communication systems. The established theory of the dc electric field induced piezoelectric effect in paraelectric phase ferroelectrics will be used in analysis of correlations between the electroacoustic parameters and material properties allowing for further improvement of the ferroelectric FBARs. Different ferroelectric materials used in the tunable FBARs as active layers will be reviewed with focus on BaxSr1-xTiO3 (BSTO) solid solution. Effects of the Ba concentration on tunable performance of the BSTO FBARs will be demonstrated including the predicted strong anisotropy in the field induced piezoelectric effect and limitation of the tunability caused by nonlinear electrostriction. Actual designs of the ferroelectric FBARs including membrane type, solidly mounted resonators and their modifications will be discussed with comparative analysis of their performances. Specific technology routs used for fabrication of the ferroelectric FBARs will be considered. Different methods of growth of the ferroelectric films used in the FBARs, including pulsed laser deposition, magnetron sputtering and sol-gel method, will be reviewed. The FBAR Q-factor will be correlated with the effects of the growth conditions and related changes in the ferroelectric film 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 the 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 will be shown that the Q-factor of the ferroelectric FBARs is limited mainly by extrinsic acoustic loss associated with wave scattering at reflection from relatively rough top interface. Measurement techniques, equivalent circuit models and design procedures developed for evaluation of both extrinsic and intrinsic parameters of the ferroelectric FBARs and materials will be considered. The novel concept of the frequency switching in the composite ferroelectric FBARs will be demonstrated. It will be shown by simulations and experimentally that the resonance frequency can be switched more than two times (from 3.6 GHz to 7.7 GHz) by changing polarity of the 5 V dc bias. This large frequency switching (more than 100%) is significantly higher than the previously reported tunabilities. This opens up new possibilities for a variety of applications in reconfigurable/adaptable microwave devices.