Ferroelectric BaxSr1-xTiO3 thin-film varactors with parallel plate and interdigital electrodes for microwave applications

Columnar BaxSr1-xTiO3 (BST) thin films with x=0.25 and 0.5 were grown by pulsed laser deposition on single crystal LaAlO3 and Pt/Ti/SiO2/Si substrates, respectively. Ferroelectric varactors were then fabricated from the BST thin films by photolithographic techniques based on parallel-plate capacitors (PPCs) and coplanar interdigital capacitors (IDCs). The microstructure, dielectric constant epsilon, loss tan delta, tunability tau, and temperature dependent characters of the BST film with IDC and PPC electrodes were investigated and compared in a wide frequency range from 1 kHz to 10 GHz. The Ba0.5Sr0.5TiO3 films exhibited larger permittivity, tunability, and loss tan delta at the level of IDC and PPC structures compared to Ba0.25Sr0.75TiO3 films. As measured by IDC and PPC electrodes, their in-plane dielectric properties also exhibited a larger permittivity, lower loss tan delta, and relative lower tunability than that of out-of-plane dielectric properties. Finally, the influence on the dielectric properties of the columnar BST grains due to the use of different types of electrodes was discussed.