Study Of Fabrication And Properties Of Ferroelectric Materials Ba_1-x-yPb_xSr_yTiO₃ For Phase Shifter Using

Ferroelectric films based shifter for phased array antenna using has attracted much more attention in recent years because it has the merits such as rapid scanning, high accuracy, low drive voltage, simple control system, multi-object tracking, and low cost, etc. In this dissertation, the design, fabricating and characterization of ferroelectric Ba_1-x-yPb_xSr_yTiO₃ (BPST) ceramics and thin films are discussed. Meanwhile, the structure and performance of the BPST thin film based coplanar waveguide (CPW) phase shifter with capacitance loaded are simulated by the Advanced Design System (ADS).The principle of ferroelectric phase shifter is discussed in this thesis. The feasibility of ferroelectric materials application on phase shifter is proved. The BPST materials are designed for the phase shifter using, and the calculation result shows that suitable BPST components are those that the subscript y is in the range of 0.3 to 0.6.The BPST ceramics are fabricated by conventional solid state reaction method. The microstructure and dielectric properties are studied. With the Pb content increasing, the BPST lattice changes from cubic phase to tetragonal one at room temperature (RT), and the lattice distortion becomes more severe. There are four BPST components, such as Ba_0.5Sr_0.5TiO₃, Ba_0.4Pb_0.1Sr_0.5TiO₃, Ba0.3Pb0.1Sr0.6TiO₃ and Ba_0.2Pb_0.2Sr_0.6TiO₃, which are suitable for phase shifter using in all the ceramics investigated. When measured at 10 kHz, the dielectric constants of above four components are 1235, 1431, 1140 and 1593, respectively. The loss tangents are 0.0029, 0.0047, 0.0041 and 0.0062, respectively. The microstructure is calculated based on the first principle. Results show that the the offset of Ti ion (denoted as ?c, which represents the displacement of Ti ion that offsets from the center of oxygen octahedron) increases with the Pb content increasing in the Ba0.7-xPbxSr0.3TiO₃ ceramics. In other BPST lattices, the total energy is lowest when ?c=0.The BPST thin films are fabricated by RF-magnetron sputtering. The sputtering techniques, dielectric performance, leakage current and elemental chemical states are investigated. When measured at 1 MHz, the RT dielectric constant and loss tangent of the Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film are 514 and 0.014, respectively. The tunability is 51% under a bias electric field of 375kV/cm, and the FOM is 36.43. With the Pb content increasing, the leakage current density increases in the BPST thin films. Under an electric field of 375kV/cm, the leakage current density of Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film is 6.7μA/cm2. The chemical bonding states of all elements that appears in the BPST thin films are investigated by XPS analysis. The results show that there are few non-perovskite bonded Ba and Ti on the surface of Ba_0.2Pb_0.2Sr_0.6TiO₃ thin film. The interior elements of Ba, Sr, and Ti are all bonded into perovskite structure, but there is few elemental lead inside of the thin film. The elements'content calculation shows that the Ba and Ti content is a little excess, while Sr content is a little deficiency both on the surface and inside of the thin films. The Pb content is consistent with the stoichiometric proportion inside of the thin films, but there is a little deficiency on the surface. The perovskite bonded oxygen content is a little deficiency both in the inside and on the surface of the BPST thin films.The structure and performance of the CPW phase shifter with capacitance loaded based on Ba_0.2Pb_0.2Sr_0.6TiO₃/MgO are simulated by the ADS. The optimal structure of phase shifter unit are as follows: the interdigitated capacitor's (IDC) finger width (s), gap between the fingers (g), long of the finger (l0), unilateral finger numbers (m) are 5μm, 10μm, 45μm, and (1or 2), respectively. The performance simulation of the phase shifter under various bias electric fields is studied. Results show that the maximal S11 and minimum S21 value of the CPW phase shifter are -17.4dB and -0.75dB respectively. The phase shifter can obtain a 330o phaseshift by changing the tunability from 0 to 15% in the frequency range of 22 to 23GHz. It's suitable for the phased array antenna using.