Thickness dependences on microwave tunable properties for (Ba, Sr)TiO3 thin films in planar capacitor structure

(Ba_0.6Sr_0.4)TiO₃ thin films of various thicknesses on sapphire-R substrate have been fabricated using the chemical solution deposition method (CSD). Inter-digitated electrode arrays in planar capacitor were formed on the surface of the BST thin film with e-beam evaporated Cu using photolithographic lift-off process. The measured capacitances of the planar-capacitor decreased with the film thickness because the electromagnetic field propagates across high permittivity BST thin film to the sapphire substrate. However, we found that permittivity of BST thin films remained large until 90 nm thick based on the electromagnetic field analysis using the finite element method. On the other hand, tunability of BST thin films decreased with the film thickness.