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 $ε$⁠, loss $tanδ$⁠, tunability $τ$⁠, 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 $tanδ$ 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 $tanδ$⁠, 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.