Ferroelectric Thin Films Materials And Application For Dielectric Phase Shifters

Tunable ferroelectric thin films for dielectric phase shifter must possess high dielectric constant tunability, low dielectric loss, good temperature stability, long lifetime, and anti-fatigue characteristic. At present, BaxSr1-xTiO3 (BST), BaZrxTi1-xO3 (BZT), and PbxSr1-xTiO3 (PST) systems have been widely studied as tunable ferroelectric materials. The barium strontium titanate (Ba0.6Sr0.4TiO3, short for BST) thin films were prepared by radio-frequency magnetron sputtering. This thesis focuses on the critical grain sizes, interface effects, preferred orientation, and reliability in order to improve dielectric properties of BST thin films. Meanwhile, the properties of BZT and PST materials were preliminary investigated. The possibility that the single-crystal substrate was replaced by the cheapness ceramics subatrate was explored. The BST thin films dielectric phase shifters were fabricated by micro-electronics technics process. Moreover, there are main results as follows.(1) By adjusting annealing temperature and time, the grain sizes of BST thin films were controlled. It is found that the crystalline critical size is about 10 nm, and the ferroelectric critical size is about 20 nm.(2) There is a transition layer at BST/Pt interface, and the thickness is about 7~8 nm. High energy Ti atomic diffusion results in a thick interfacial transition layer. It is found that the transition layer was diminished to about 2~3 nm thickness by reducing the initial RF sputtering power. Moreover, the symmetry ofεr-V curve of BST thin film is enhanced from 52.37% to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (△Pr), and that of coercive field (△EC) are remarkably improved.(3) The structural properties of BST thin films are strongly dependent on the direction of the substrate. The BST thin films grown on LaAlO3 (100) displayed (h00) preferred orientation, while the films grown on LaAlO3 (110) indicated (110) preferred orientation. BST films grown on LaAlO3 (110) had higher tunability, better figure of merit, and lower leakage current density than that grown on LaAlO3 (100).(4) BST thin films grown on Pt/Ti/LaAlO3 (100) substrates displayed excellent reliability. From -30°C to 130°C, dielectric constant, loss tangent and tunability (at 300 kV/cm) are slightly increased with increasing temperature. From 20 Hz to 1 MHz, dielectric constant, loss tangent and tunability of BST thin films have a little change. After a hundred million reversals, the tunability of BST thin film and the dielectric loss hardly change.(5) When x = 0.20, the crystal structures of the BaZrxTi1-xO3 ceramics are very close to cubic. However, the crystal structures are tetragonal when x < 0.20. The temperature dependence of the dielectric constant was studied and an enhanced diffuse phase transition behavior is found to be caused by the increased Zr content. The decreases of coercive electric field and remanent polarization were the result of increase of Zr/Ti ratio in BaZrxTi1-xO3.(6) BaZr0.1Ti0.9O3 films displayed a highly (h00) preferred orientation and a good cube-on-cube growth on the LaAlO3 (100) substrate, while there is no obvious preferential orientation in BaZr0.2Ti0.8O3 thin films. The BaZr0.1Ti0.9O3 films possess larger grain size, higher dielectric constant, larger tunability, larger remanent polarization and coercive electric field than that of BaZr0.2Ti0.8O3 films. However, BaZr0.1Ti0.9O3 films have larger dielectric losses and leakage current density. The results suggest that Zr4+ ion can decrease dielectric constant and restrain non-linearity.(7) At room temperature, the PbxSr1-xTiO3 ceramics were tetragonal phase when x≥0.45. It is found that the PST ceramics for x = 0.40 and 0.45 exhibit antiferroelectric-like behavior. For Pb0.45Sr0.55TiO3 ceramics, the antiferroelectric-like behavior enhanced as sintering temperature and maximum external electric field increased. Moreover, increasing excess lead content leads to abnormal hysteresis loops. The antiferroelectric-like behavior may be described in terms of the strongly stressed hybrid structure, domain pinning and oxygen vacancy.(8) The surface of ceramics substrates were modified by coating glass glaze. The microstructure and dielectric properties of BST thin films deposited on modified substrates are almost same with that of thin films deposited on LaAlO3 single-crystal substrates. So, the modified substrate is a possible alternative to single-crystal substrate and polished ceramics substrate.(9) The dielectric phase shifter with continuous 360°phase shift was successfully fabricated by optimum BST ferroelectric thin films and patterned technics process. At 16.5 GHz and 40 V, the phase shift and insertion loss are about 405°and -16 dB respectively.