Preparation And Investigation Of Manifold-domain-wall-variant BST Thin Films

Tunable capacitor devices,i.e.varactors,as one of the representatives of microwave tunable devices,have been widely used in tunable networks of communication systems.As the core of the varactors,the performance of the dielectric tunable materials plays a decisive role in the dielectric properties.As a ferroelectric material,barium strontium titanate(Ba_1-xSr_xTi O₃,BST)can change dielectric constant as a function of applied electric field.Coupled with high dielectric constant,it becomes a popular material for tunable dielectric films in varactors,and has broad research value and application prospects.In general,the BST in the varactors is in the paraelectric phase.Because there are domain walls inside the ferroelectric phase.In AC circuits,especially in high-frequency circuits,the movement of the internal domain walls of the thin films will significantly increase the overall loss of the devices and reduce the dielectric performance.However,in recent years,some scholars have discovered a manifold-domain-wall-variant structure.Being different from the traditional domain wall,it not only has lower energy loss when moving,but also can resonate at a suitable electric field,which promotes the transformation of the direction of the internal electric dipole moment,and even shows high quality factors in the high-frequency environment of 10 GHz,which has great application potential.After analyzing and summarizing the formation requirements of manifold-domain-wall-variant structure,BST dielectric films were prepared on（110）Gd Sc O₃ substrates by RF magnetron sputtering method.By studying the influence of the process conditions on the microstructure of BST films,high-quality epitaxial BST films were obtained.The films were prepared into interdigital electrode varactors,and the dielectric properties of the them with different BST film thicknesses in the range of 2-8 GHz were studied.The main research contents and results are as follows:1.The BST thin films were deposited by RF magnetron sputtering system.The effects of substrate temperature,sputtering power,oxygen-argon ratio,sputtering pressure and post-annealing temperature on the structure and surface of BST film were studied.The preparation process conditions of BST film were optimized,and epitaxial BST film with good adhesion and few dislocation defects was prepared.The deposition rate of the process is 30 nm/h.The epitaxial relationship between the films and the substrates is[001]_BST|[110]_GSO and[010]_BST|[001]_GSO.2.The interdigital electrodes were prepared by photolithography-peeling method.The dielectric properties of dual-port interdigital varactors at 2-8 GHz were measured by vector network analyzer,and the influence of dielectric layer film thickness on the performance of variable container was analyzed.The sample with thinner dielectric layer had a large Q value and a smaller capacitance,while the opposite was true for sample with thicker dielectric layers.The Q value of both samples peaked around 2.5 GHz and7.5 GHz,which proves that the thin film has the manifold-domain-wall-variant structure.Affected by parasitic parameters,the capacitance of the varactors showed a proportional relationship with frequency.