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Study on mechanism and nonlinear properties of BST thin films

Barium strontium titanate (BaxSr1-xTiO3, BST) thin films are potential materials for the applications of microwave devices such as dielectric phase shifters and tunable filters due to their high dielectric nonlinearity, low leakage current and adjusting Curie temperature. The microstructures and dielectric properties of BaxSr1-xTiO3 (x = 0.45 ~ 0.9) ceramics, film thickness uniformity, microstructures (including composition, crystal structure and domain, etc) and size effect on dielectric nonlinearities of BST films, model for dielectric nonlinearity of ferroelectric films, design and fabrication of phase shifters were investigated. Moreover, there are a series of results as follows: On condition that the center of the substrate is not coincident with that of the target, the magnetron sputtering system was studied. Under the conditions of planetary movement of a substrate underneath a magnetron sputter cathode, a model on the relation between the film thickness and the parameters, such as rotation speed, revolution speed and the distance from target to substrate, was obtained. The relative film thickness distribution can be expressed by T = ∫ L d (x,y)ds=∫0t d(ξ (t),ψ(t))ξ'2(t)+ψ'2(t)dt. It is found that the relative deviation of film thickness distribution is 2.8% within a diameter of φ120 mm when the ratio of the rotation speed to revolution speed and the distance from target to substrate are 5.3 and 72 mm, respectively. The species and sizes of domains in BST thin films were investigated by piezoresponse force microscopy (PFM). The results show that there are 90° domains. It is found that the critical grain size from a multi-domain to a single-domain is about 31 nm. The size effects on nonlinearity of BST thin films were investigated and the results show that the grain size and film thickness play an important role on nonlinearity of ferroelectric films. The dielectric constant and the tunability of BST thin films increase as the grain size and/or film thickness increase. The tunability and the coercive electric field of BST films with mono-domain configuration are intensively affected by grain size. However, there are no obvious grain size dependences of the tunability and the coercive electric field in BST films with multi-domain configuration. Based on above experimental researches, the mechanism of dielectric nonlinearity of ferroelectrics was investigated. Firstly, the simple and useful polynomial functions of P(E) and e(E) were presented. Based on the characteristics of hysteresis loops and e-E curves in ferroelectric materials, boundary conditions and original conditions were obtained, therefore the coefficients of the functions were expressed by Ps, Pr, Pm, EC and EV. Based on these results, a model for dielectric nonlinearity of ferroelectric crystals was established. Secondly, utilizing an ideal geometric model and equivalent circuits of ferroelectric ceramics, a model including grain size and grain boundary width, for dielectric nonlinearity of ferroelectric ceramics was established. And then the interfacial barriers were modeled in terms of diode equivalent circuits, therefore a model including grain size, film thickness and interfacial layer thickness, for dielectric nonlinearity of ferroelectric films was established. Finally,the accuracy of the model prediction was quantitatively verified by the data and curves from literatures. The results proved that the above model can describe the dielectric nonlinearity of ferroelectric materials, including ceramics and films, and the size effects on nonlinearity. The design, fabrication and measurement of film phase shifters were investigated. Utilizing high-frequency electromagnetic emulators such as ANSOFT HFSS9.0 and ADS, the structure of the dielectric phase shifter with loaded parallel capacitors has been designed, and its parameters have been obtained. The film phase shifters with orderly electrodes were fabricated by peel-back technique. The microwave properties of film phase shifters were measured by vector network analyzer. The phase shift and insertion loss are about 180° and 9.7 dB respectively, with an electric voltage of 40 V at 35 GHz.