Low loss and high tuning barium strontium titanate (BST) thin films

A strain-relieved and epitaxially grown Ba_1-xSr_xTiO₃(BST, x = 0.3 0.6) films on (100) MgO were prepared by pulsed laser deposition. The microstructure of the thin films, surface morphologies, grain sizes and film thickness as well as varactor electrode quality and gap size were characterized by optical microscope and scanning electron microscopes (SEM). X-ray diffraction (XRD) was used for chemical compositions and crystal structure and orientations analyses. Interdigitated varactors at different finger gap sizes were fabricated by lift-off process. Dielectric tuning, varactor capacitance and dielectric loss were measured at frequencies at 2 GHz. Relationship between the electrical properties and microstructures of the deposited thin films were investigated. Tunability of PLD BST thin film decreased with increasing interdigitaed finger gap sizes; however, Q values increased with increasing gap sizes. Thus, the inverse relationship between dielectric tuning and dielectric Q was existed, but highly improved (45% tuning and 110 Q at 10 μm gap size).