Suppression of Acoustic Resonances in All-Oxide Varactors
Barium strontium titanate (BST) thin-film varactors promise very good performance in RF frontends in terms of low loss and high tunability. However, their application is commonly limited to lower GHz frequencies. For higher frequencies, acoustic resonances drastically reduce the device quality in metal-insulator-metal (MIM) structures under bias voltage. In this work, this limitation is overcome by replacing the metal electrodes with conducting oxides that structurally match to BST and, therefore, avoid acoustic impedance mismatch. A detailed analytic model is derived, incorporating electric and acoustic behavior. Four samples with an oxide bottom electrode and varying BST thickness are characterized and fitted by the derived model with very high accuracy. Each shows a significantly reduced degradation due to acoustic loss. A model-wise comparison of stacks with metal and oxide electrodes demonstrates the strong benefit of all-oxide varactors, the possibility of complete suppression of acoustic resonances.