Ba_xSr_ (1-x) of TiO_3 thin films with tunable bulk acoustic wave resonator
|School||University of Electronic Science and Technology|
|Course||Electronic Science and Technology|
|Keywords||thin film bulk acoustic resonators (FBAR) solidly mounted resonators(SMR) Tunable Frequncy BaxSr1-xTiO3thin film FBAR filter|
With the development of wireless communication technology, it is essential to require a filter which can be tuned to accommodate operation at different frequency bands for a mobile phone with multiple functions. Tunable FBARs using the traditional piezoelectric materials AlN, and ZnO are not very promising due to very low tuning or tuning speed, power consumption issues. Tunable FBARs of PZT with relatively high tunability have been reported; but one problem for practical applications is the hysteresis due to the polar phase of the material. Compared with the traditional materials, BaxSr1-xTiO3(BST) film is in paraelectric phase at room temperature without piezoelectricity; however, under an applied dc bias, BST film induces piezoelectric effect due to electrostriction, which is nonlinear with respect to the mechanical deformation. Tunable FBARs, filters, and RF switches have been reported using BST film based on this feature, which makes integrated tunable circuit components possible for wireless communication systems with low loss, high Q factor, and low control voltage.(1) The estimation of the film bulk acoustic resonators FBAR is implemented in the ADS simulation software. The BST FBAR with3GHz resonant frequency is designed using this model. The Simulation results show that:Bragg reflectors composed of3pairs of Mo and SiO2alternating high and low acoustic impedance layers can play a good role of restraining acoustic leakage. The influence of Si substrate can be ignored.(2) The solidly mounted thin film bulk acoustic resonator (SMR) based on Ba0.5Sr0.5TiO3film was fabricated using SiO2/Mo Bragg reflectors. Under60V dc bias, the maximum electromechanical coupling coefficient of FBAR was up to8.09%, which can be comparable to AlN and ZnO films. The relative tunability of shifts at series resonance frequency is around-2.3%.(3) Tunable film bulk acoustic resonators based on Ba0.5Sr0.5TiO3with differentstructure and size was studied. The resultes show that Ohmic losses and the parasitic inductance of the test platinum lead is the main a significant negative effect on FBAR. BST FBARs with small effective area show stronger electrostrictive effect, which can be atttibuted with static capacitance. (4) Tunable film bulk acoustic resonators based on Ba0.5Sr0.5TiO3with different metal material and the thickness of top electrode were fabricated. The influence of ohmic loss and mechanical loads of top electrode on FB AR was discussed. Both ohmic loss and mechanical loads of top electrode have a significant negative effect on FBAR. The tunability and quality factor are suppressed by the ohmic loss and mechanical load of top electrode. However, the ohmic loss of electrode is a significant energy loss mechanism at the series resonance, while mechanical energy loss is a main energy loss mechanism at the parallel resoncance.(5) The parameters of resnantor were extracted from mason model. A tunable BST FBAR filter was designed. The application of40V dc bias results in the filter being place in an on state with a center frequency of3.0GHz, a50MHz bandwidth,20dB of band attenuation and1.02dB of insertion loss.