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

Abstract

The design of a BaSrTi3 (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 SrMoO3 (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.