ABSTRACT
Mode-switchable ferroelectric thin-film bulk acoustic resonators (FBARs) are presented. Such resonators operate based on a dynamic nonuniform effective piezoelectricity in composite multilayer ferroelectrics with large electrostriction coefficients, like barium strontium titanate (BST). Harmonic resonance modes ( nfo ) of a multilayer ferroelectric bulk acoustic wave (BAW) resonator can be selectively excited with an electromechanical coupling coefficient ( Keff2 ) equal to the fundament mode, which is contrary to the trend Keff2 â 1/n2 exhibited by conventional piezoelectric BAW resonators. Such a device can selectively be set to resonate at its different resonance harmonics by generating a pattern of nonuniform piezoelectric coefficient proportional to the stress field of each mode with an application of a proper set of dc control voltages applied across the ferroelectric layers. Such a resonator allows for the design of a new class of band-switching filters. As an experimental validation, a mode-switchable FBAR and a band-switching ladder-type filter based on a bilayer ferroelectric BST structure are designed and fabricated for the first time. The bilayer BST FBARs not only can be switched ON or OFF but also by choosing different bias configurations, two resonance modes at 2 and 3.6 GHz can be selectively excited having Keff2 of 8% and 7%, respectively.
ABSTRACT
Reconfigurable bulk acoustic wave filters provide a number of advantages for wireless communication systems, including compact size, cost effectiveness, and less complexity. This article presents the design methodology of an intrinsically switchable and bandwidth reconfigurable film bulk acoustic resonator (FBAR) filter, employing the electrostriction in ferroelectric barium strontium titanate (BST). Two examples of ferroelectric BST-based filters are designed and fabricated as a proof of concept. Under the application of a dc bias voltage, the fabricated switchable filters exhibit a bandpass response with a fractional BW of 3%. By changing the state of the ferroelectric BST FBARs through applied dc bias voltages, the bandwidth of the filters is adjusted. Without any bias, the filters switch OFF to provide isolation between the input and output ports. BST-based bandwidth reconfigurable filters can potentially reduce the number of required filters and switches in future multiband RF front ends.
ABSTRACT
Intrinsically switchable thin film bulk acoustic resonator (FBAR) filters based on ferroelectric barium strontium titanate (BST) are presented. A 1.5-stage -network intrinsically switchable FBAR filter unit cell with a 3-dB fractional bandwidth of 3% at 2 GHz is systematically designed, simulated, and fabricated. The minimum insertion loss (IL) for the filter unit cell is 2.25 dB, representing the lowest IL reported for BST bulk acoustic wave filters to date, which is mainly due to its high BST resonators. Two 1.5-stage filter unit cells are connected in series to form a 2.5-stage filter, providing more than 25 dB of out-of-band rejection and OFF-state isolation between the input and the output ports. The measured input third-order intercept point (IIP3) of the 2.5-stage filter is 47 dBm. Furthermore, the footprint of the fabricated filters is notably small, due to the high permittivity of BST.
ABSTRACT
This paper presents a ladder-type bulk acoustic wave (BAW) intrinsically switchable filter based on ferroelectric thin-film bulk acoustic resonators (FBARs). The switchable filter can be turned on and off by the application of an external bias voltage due to the electrostrictive effect in thin-film ferroelectrics. In this paper, Barium Strontium Titanate (BST) is used as the ferroelectric material. A systematic design approach for switchable ladder-type ferroelectric filters is provided based on required filter specifications. A switchable filter is implemented in the form of a BST-on-Si composite structure to control the effective electromechanical coupling coefficient of FBARs. As an experimental verification, a 2.5-stage intrinsically switchable BST-on-Si composite FBAR filter is designed, fabricated, and measured. Measurement results for a typical BST-on-Si composite FBAR show a resonator mechanical quality factor (Q(m)) of 971, as well as a (Q(m)) × f of 2423 GHz. The filter presented here provides a measured insertion loss of 7.8 dB, out-of-band rejection of 26 dB, and fractional bandwidth of 0.33% at 2.5827 GHz when the filter is in the on state at a dc bias of 40 V. In its off state, the filter exhibits an isolation of 31 dB.
ABSTRACT
This paper presents a voltage-controlled, highquality factor (Q) composite thin-film bulk acoustic resonator (FBAR) at 1.28 GHz. The composite FBAR consists of a thin layer of barium strontium titanate (BST) that is sandwiched between two electrodes deposited on a silicon-on-insulator (SOI) wafer. The BST layer, which has a strong electrostrictive effect, is used for electromechanical transduction by means of its voltage-induced piezoelectricity. The silicon layer, with its low mechanical loss, increases the Q of the resonator. The composite FBAR presented here exhibits Qs exceeding 800 with a resonance frequency and Q product (f × Q) of 1026 GHz.
