All-Oxide Thin Film Varactor: From Test Structure to SMD Component

The design of a BaSrTi
₃
(BST) thin film varactor in metal-insulator-metal configuration is presented, prepared for surface mount device (SMD) package integration. Instead of metal, the oxide conductor SrMoO
₃
(SMO) is used as bottom electrode. This material shows not only metal-like conductivity (comparable to platinum), but additionally structural similarity to BST. This allows the growth of very thin, high quality epitaxial ferroelectric layers, which provide a higher quality and full tuning at low voltage levels. The fabrication, on-wafer characterization and modeling of the integrable varactor design is explained. The measured varactor performance with Q = 38 and tunability τ = 28.6% at 1GHz and bias voltage 3.7 V is degraded with respect to material performance. The reasons for this degradation are analyzed in detail, as Q = 75 and τ > 70 % were expected from material characterization. De-embedded results and the model for improved metal thicknesses predict a device Q = 95 and indicate the general suitability of this varactor concept for the application in mobile communication.