Composite thin films consisting of fine-grained barium strontium titanate for tunable microwave devices

It has been reported that grain size has significant influence on the dielectric properties of barium strontium titanate (BST) thin films. In this paper we have proposed a multilayer structure in which BST layers are separated by thin MgO layers which are expected to function as barrier to atomic diffusion so that the grain size of BST could be confined. By simulation we have found that for a device with a coplanar/composite film/substrate configuration, the introduction of MgO can effectively change the dielectric constant of the composite while the tunability is not significantly affected, provided the amount of MgO is smaller than 25%. A prototype device was prepared to demonstrate the feasibility of this idea. By X-ray diffraction measurements it is revealed that BST and MgO coexist in the composite. The dielectric property of the composite film was determined and it was found that the film has a tunability of up to ~7% at 500 MHz and under an electric field of 13.3 kV/mm.