Tunable Film Bulk Acoustic Wave Research Articles

Highly Tunable Film Bulk Acoustic Wave Resonator Based on Pt/ZnO/Fe65Co35 Thin Films.

We have developed a highly tunable film bulk acoustic wave resonator (TFBAR) using magnetostrictive (MS) Fe65Co35 thin films in acoustic layer stack. The resonator acoustic layer stack consists of Pt/ZnO/Fe65Co35 layers to tune the devices. Due to ∆E effect, TFBAR resonance frequency was up-shifted ~106.9 MHz (4.91%) in the presence of 2-kOe magnetic field. From experimental measurement, ∆E enhancement was estimated to be ~35 GPa. Further, it is observed that return loss ( S11 ), phase response, and quality factor were improved in the presence of magnetic field. This improvement is due to the field-induced stiffness in the magnetic layer. Equivalent-modified Butterworth-Van Dyke (mBVD) circuit model was developed and fit with the experimental data, and circuit parameters were extracted. The proposed resonator is compact, low loss, power efficient, and highly tunable. This method also facilitates a new method of tuning FBAR devices using MS thin films.

Enhanced tunable performance of high Q-factor BaxSr1−xTiO3 film bulk acoustic wave resonators

Emerging intrinsically tunable film bulk acoustic wave(BAW) resonators allow the development of new generation reconfigurable and agile microwave circuits. In this paper, we demonstrate the enhancement of tunable performance of the high Q-factor BaxSr1−xTiO3 BAW – solidly mounted resonators (BAW–SMR) by varying Ba concentration. The Ba0.5Sr0.5TiO3 BAW–SMR reveal tunability of series resonance frequency up to 2.4%, electromechanical coupling coefficient up to 7.5% and rather high Q-factor, up to 250 at 5.3 GHz. Correlations between the measured electroacoustic parameters are analyzed using the theory of dc field-induced piezoelectric effect in paraelectric phase ferroelectrics. Higher coupling coefficient and tunability of resonance frequency of the Ba0.5Sr0.5TiO3 BAW–SMR are associated with higher tunability of permittivity. Strong anisotropy in field-induced piezoelectric effect is predicted with highest coupling coefficient in (001) direction of the BaxSr1−xTiO3 films. It is also shown that the tunability of series resonance frequency of Ba0.5Sr0.5TiO3 BAW–SMR is limited by relatively high and negative nonlinear electrostriction coefficient which is found to be m ≈ −4·1010 m/F. The BAW–SMR Q-factor is limited significantly by extrinsic acoustic loss associated with wave scattering at reflection from relatively rough top interface. The results of analysis show possible ways of further improvement of the performance of tunable BAW–SMR.

Microwave Response of BiFeO3 Films in Parallel-Plate Capacitors

Dielectric response of the BiFeO3 films in a parallel-plate capacitor configuration is studied in the frequency range 1 MHz–30 GHz and under dc electric field up to 45 V/μm in view of their application in tunable film bulk acoustic wave resonators. The observed relatively high permittivity 130 without remarkable frequency dispersion is explained by contribution of the domain wall vibrations. It is shown that the substrate induced strain and Maxwell-Wagner contributions are negligible. The measured dielectric response allows estimation of the BiFeO3 sound velocity and acoustic impedance as 3100 m/s and 26 · 106 kg/m2s, respectively.