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Tunable ferroelectric FBARs: An overview
A. Vorobiev*, and S. Gevorgian
Department of Microtechnology and Nanoscience, Chalmers University of Technology
Kemivagen 9, Gothenburg, Sweden, SE-412 96
*e-mail: andrei.vorobiev@chalmers.se
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.
1. Tuneable film bulk acoustic wave resonators, S. Gevorgian, A. Tagantsev, A. Vorobiev, London,
Springer (2013).
2. Intrinsically switchable thin film bulk acoustic wave resonators, A. Vorobiev and S. Gevorgian,
Applied Physics Letters, 104, 222905 (2014).
Curriculum Vitae Andrei Vorobiev received the M.Sc. degree in physics of semiconductors and dielectrics from the Gorky State University, Gorky, Russia, in 1986 and Ph.D. degree in physics and mathematics from the Institute for Physics of Microstructures of Russian Academy of Sciences (IPM RAS), Nizhny Novgorod, Russia, in 2000. Since 2001, he is at Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg, Sweden. In 2008 he received title of Associate Professor in physical electronics. He has authored or coauthored over 60 journal articles, over 70 conference contributions (4 invited, including EMF-­‐2011, Bordeux), 3 books and 4 patents. His research interests are in the area of development of models and applications of emerging functional materials and phenomena in microwave components and devices based on multiferroic and ferroelectric thin films and, recently, graphene, including development of materials, thin film and microtechnology fabrication/processing steps and experimental investigation.