Ferroelectric thin films are currently being used to develope tunable microwave devices based on the electric-field dependance of the dielectric constant. We fabricated coplanar waveguide(CPW) resonator to investigate the basic properties of the $Ba_xSr_{1-x}TiO_3$ (BST) thin films. And on the basis of resonator properties, we simulated and made CPW phase shifters.
The geometry of the multilayered CPW device structure was simulated using the Zeland's IE3D software to model the behavior of impedance, effective dielectric constant, etc. Microwave tunable devices were fabricated with BST thin films and gold. BST thin films of the thickness of 0.5㎛ was deposited on MgO (100) single crystal substrate by RF magnetron sputtering method. Gold electrode of 1㎛ thickness was prepared by the DC magnetron sputtering method and devices was patterned successively by photo-lithography and lift-off (or ion milling) process.
The resonator using BST thin films was designed for center frequency of 12GHz. But the measurement value of the resonant frequency was 14.695GHz. As a result of this value, we calculated that the dielectric constant of BST thin film was ~297.
Using the dielectric constant of BST thin film calculated in resonator, we designed phase shifters and fabricated it. Line type phase shifter was simulated to have maximum 10° of phase difference and below 0.6dB of insertion loss. The measurement result was similar to the simulation result. To increase the phase difference, we designed the novel type phase shifter which has slits on ground plane. A novel type phase shifter has greater phase difference than line type one.