The Study On The Dielectric Properties Of Ba_xSr_1-xTiO₃ Thin Film Applied To The Tunable Film Bulk Acoustic Wave Resonator

With the rapid development of wireless communication technologies to the direction of multi-band, the higher performance is desired for the tunable FBAR. Not only is the high tunability needed, but also the low insertion loss is required. For the FBAR manufactured by the traditional piezoelectric materials such as AlN, and ZnO etc., the tunability is low and power handling capability is poor, which could not meet the practical application. Compared with the conventional piezoelectric materials,the ferroelectric thin film BST not only has high tunability, and also could withstands high power, apart from these, its dielectric loss is low, so the BST film is very suitable for the tunable FBAR applications.In order to reduce the dielectric loss and improve the dielectric properties of the BST films, the paper focus on the research of compositionally graded and the doping BST films, the main contents and conclusions are as follows:Ba0.3Sr0.7TiO3 thin films with the thickness of 450 nm were prepared on the Pt/Ti/SiO2/Si substrate by the sol-gel method,and then were sintered at different temperatures. The results showed that the Ba0.3Sr0.7TiO3 thin film sintered at 750? had the lowest dielectric loss, the highest dielectric constant and figure of merit,which had optimal dielectric properties.In the compositionally graded BST films, the performance of each mutation can be eased, which could effectively improve the performance of BST films. Single composition BaxSr1-xTiO3 films(x =0.3, 0.5, 0.7) and the upgraded film(a direction away from the substrate, x changes from 0.3 to 0.5 and then to 0.7) and the downgraded film(x changes from 0.7 to 0.5 and then to 0.3) were sintered at 750?.The test results showed that the Ba0.5Sr0.5TiO3 film achieved the highest tunability of 17.4% under the electric field of 110kV/cm. At room temperature, the operating temperature of Ba0.5Sr0.5TiO3 film is closer to its Curie temperature. The BST film which works near the Curie temperature always has a higher tunability. The downgraded BST film has the highest dielectric constant of 323, the lowest dielectric loss of 0.037 and the highest figure of merit of 4. Compared to the upgraded BST film, the relative displacement of Ti and O ions in the oxygen octahedron and the polarization of electric dipoles in the downgraded BST film are larger, so the dielectric properties of downgraded BST film outperforms the upgraded BST film.The ionic radius of Rb is very close to the ones of Ba and Sr ions. Rb acting as the acceptor could neutralize the electrons generated from the oxygen vacancies to reduce the dielectric loss. Moreover, Rb-doped BST films has not been reported yet, so we prepared Rb doped Ba0.3Sr0.7TiO3 thin film with the doping amount from 0mol% to 8mol% by using the sol-gel method. It was found that the 4 mol% Rb-doped BST film showed the highest density, the highest dielectric constant of 292, the lowest loss of 0.028, and the highest FOM value of 8.1.