Fabrication and characterisation of barium strontium titante thick film device structures for microwave applications

Tunable capacitor elements for microwave phase shifter applications have been fabricated by screen printing barium strontium titanate (BST) films on alumina substrates. A Ba0.70Sr0.30TiO3 composition was chosen for the initial devices as it has been shown to exhibit high tunability at room temperature. A vertical capacitor test structure involving a Pt lower electrode, BST film and Ag top electrode has been used throughout the work. The tunability and figure of merit (phase shift/dB of insertion loss) at 2-3 GHz were found to be strongly dependent on the sintering temperature of the BST layers, with properties improving as the sintering temperature was increased. However, for sintering temperatures >1280 °C, the device properties could not be measured, possibly indicating a problem with the lower Pt electrodes. In order to reduce the sintering temperature required for densification, test structures have also been fabricated using other BST compositions to which sintering aids have been added. Finally, a reflection-type phase shifter (RTPS) based on the capacitor test structure and optimised processing conditions is presented with microwave measurements results.