Thin film Barium-Strontium-Titanate Parallel-Plate varactors integrated on low-resistivity silicon and saphhire substrate

Barium-Strontium-Titanate (BST) thin film based ferroelectric varactors are designed at specific capacitances under 0V dc bias on CMOS compatible low-resistivity silicon substrate. The BST varactor device operation is based on the nonlinear dielectric tunability of BST thin film sandwiched between two metal plates in a revised conductor-backed coplanar waveguide (CBCPW) transmission line configuration. The varactor capacitance at 0V dc bias is determined by the overlap area between the CPW signal line in the top metal electrode and a tapered shunt line in the bottom electrode. Therefore a series of devices with unbiased capacitances ranging from 0.8pF to 4.8pF were designed and fabricated based on changing their corresponding overlap areas according to the generic parallel plate capacitance equation. A schematic model was also utilized to extract the designed and measured capacitances. The relationships between the sizes of overlap areas and the extracted capacitances from the electromagnetic and schematic models are demonstrated by a reasonable agreement with the experimental measurements from fabricated devices. Devices were also designed and fabricated on sapphire substrate with three layout variations aiming to modify the parasitic resistance.