Characterization of the Field-Dependent Permittivity of Nonlinear Ferroelectric Films Using Tunable Coplanar Lines

This letter demonstrates a scheme to characterize very precisely the field-dependent permittivity of nonlinear ferroelectric films using inhomogeneously tuned coplanar transmission lines. For the characterization, tunable coplanar waveguides on a screen-printed Ba_0.6Sr_0.4TiO₃ (BST) thick film are realized and measured at radiofrequencies to determine the geometry-dependent tunability of the capacitance per unit length C´. To characterize the nonlinear tunability of the BST-thick film, these C´-values are compared with capacitance values simulated by the finite-difference time-domain method. Avoiding numerical instabilities of the iterative extraction procedure, the convergence of the extracted tunability is validated by three distinct geometries on the same BST substrate. The extracted tunability of the BST-thick film is 67% for an electric field-strength of 20V/mum. The precise knowledge of the field-dependent permittivity enables a distinct tunability optimization of frequency-agile radio frequency devices based on nonlinear ferroelectric films