Characteristics of (Ba,Sr)TiO₃ thin films for microwave tunable device applications

Ferroelectric (Ba_0.5Sr_0.5)TiO₃ (BST) thin films with thicknesses of 500 nm were deposited on LaAlO₃ (LAO) substrates by RF magnetron sputtering at 800 °C. The BST films were characterized for structure by using X-ray diffraction (XRD). The surface morphologes and the thicknesses of BST films were characterized by using atomic force microscopy (AFM) and field-emission scanning electron microscope (FESEM), respectively. We measured the dielectric properties at microwave frequencies (1-3 GHz) by using a symmetrical stripline resonator with shorted ends at room temperature. The tunability of the as-deposited and the annealed BST films were 1 % and 27 % at a field of 40 kV/mm and values of tani of 0.0038 and 0.0050, respectively. Through a post-annealing process, we were able to improve the dielectric constant and tunability, although the dielectric loss was slightly increased. The change in the dielectric properties after annealing could be attributed to a change in the film strain.