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Ferroelectric thin film based multilayer structure for acoustic devices with electrical multiband switching ability
Keywords: Ferroelectrics, Thin films, TFBAR
Thin film ferroelectric bulk acoustic resonators (FBAR) offer a significant potential in modern microwave electronics. The key features of the FBARs are their ultra-small size and the ability to work at frequencies up to tens of gigahertz. They are widely used in miniaturized filters for communication and navigation systems. Modern microwave FBARs are multilayer thin film structures containing one piezoelectric layer (AlN, ZnO)[1–3]. The operating frequency of these resonators is determined by the thickness of the structure and by the elastic properties of the layers, and cannot be electrically switched or effectively tuned. Therefore, the development of FBARs with capability of electrical control of their resonance frequency is a challenge whose solution could lead to a significant improvement of modern filter devices and microwave systems]4]. A possible solution of this problem is to replace the piezoelectric materials that are traditionally used for FBAR fabrication with ferroelectric materials in the paraelectric phase. A good candidate is barium-strontium titanate (BaxSr1-xTiO3, BSTO)[4—11], due to its electric field induced piezoelectricity with relatively high values of electromechanical coupling coefficient that allows for the design of tunable and switchable FBARs. Furthermore, by varying the Barium to Strontium stoichiometric ratio, one could alter the BSTO material properties and enhancing either the piezoelectric coefficient (by increasing the Barium content) or the electrical/acoustical quality factor (by increasing the Strontium content). The BSTO has a perovskite crystal structure with Ti atom in the center of lattice cell. In the paraelectric phase (i.e. at temperatures above Tc, where Tc is Curie temperature) and in the absence of an applied electric field, Ti is in the central position, the system is symmetrical, and BSTO does not have piezoelectric properties. In the ferroelectric phase or under external electric field, Ti shifts from the central position, which leads to the loss of symmetry and the appearance of the piezo-electric effect. In the case when the shift is caused by external electric field, the piezoelectric effect is called an induced piezo-effect. An undesirable feature of the ferroelectric materials is the strong temperature dependence of permittivity (T) near the temperature of their ferroelectric-paraelectric phase-transition. However, in BSTO films, with compositions x = (0.10.5), under dc electric fields Edc40V/m (corresponding to the maximum values of the electromechanical coupling coefficient), this undesired effect is suppressed. Let us assume that electric field is a sum of two components E = Edc+Emw, where Edc – constant bias, Emw – microwave (MW) signal. At |Emw| << |Edc|, the S microwave response can be approximated as linear, and the value and the sign of the derivative ∂S/∂E define the piezo-electric coefficient (d) at different Edc operating points. Beside the d variations under Edc, the other field-dependent material parameters, such as permittivity (eps) and stiffness (c), are changed under Edc that results in the tuning of the resonance frequency of BSTO FBAR[5–11]. Both theoretical and experimental results predict a maximum electrical tuning up to 5% of the FBAR’s operating frequency by varying the magnitude of the applied bias field[5–10]. However, in practical applications, tuning the resonant frequency by 5% is problematic due to the significant variation of the input impedance of the FBAR that inevitably leads to a mismatch of the microwave circuit. However, for a set of applications the use of this phenomena for small (up to (1-2)%) frequency tuning is desirable[9]. In this paper, another approach based on the use of induced piezoelectric phenomena in ferroelectrics is considered. The main idea is the use of multilayer thin film FE structure with conductive electrodes between layers, which make possible to apply Edc of different polarities and magnitudes to each FE layer or groups of FE layers. This results in the ability to change the sign and value of its piezo-coefficients and allows enormous switching of the FBAR’s operation frequency. One of the main requirements for frequency switchable resonators is the presence of excitations at one selected resonance mode and suppression of other modes. So, further in the paper, the principle of selectivity for a resonator with an arbitrary number (n-layer) of FE active layers is formulated. A so-called “criterion function” is proposed, which makes possible to determine the magnitudes and polarities of the control voltages applied to each FE layer to provide the excitation or suppression of operating resonance modes. As an example, the expected multiband switching ability of a FBAR with four active FE layers is considered. We validated the proposed model by comparison of the results of our calculation with the data from previous experimental works [12-14]. Finally, the possible applications of two layer structures for switchable MW high overtone bulk acoustic resonator (HBAR), binary and quadrature phase-shift keying (BPSK, QPSK) modulators are suggested.
Dr Peter K Petrov, Principal Research Scientist
Imperial College London
London, London

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