Coplanar Waveguide Using Ferroelectric Thin Oxide Film: Dielectric Constant

Abstract

Coplanar waveguide (CPW) transmission lines were fabricated on thin ferroelectric Ba1 − xSrxTiO3 films for tunable microwave applications. The growth of the ferroelectric oxide films was accomplished by a pulsed laser deposition with a partial oxygen background. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.045–20 GHz with −40–40 V of dc bias. A large phase shift angle of ∼ 120∘ at 10 GHz was observed from the CPW (gap = 4μm, length = 3 mm) with a 40 V of dc bias change. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameter by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film exhibits a gap dependency; dielectric constant (990–830) decreases with increasing gap size (4–19 μm, respectively). By adjusting the filling factors of the film, a ‘constant’ dielectric constant of BST film is found to be 810 ± 5.