Evaluation of Microwave Capacitors and Tunable Waveguides on (Ba, Sr)TiO₃ Films Deposited on Sapphire Substrates

Abstract

Tunable microwave devices have been widely investigated because next generation information communication equipment will require accommodation of several communication protocols and frequencies. Barium strontium titanate (Ba,Sr)TiO 3 (BST) thin films are expected to be applied in such devices because of their nonlinear dielectric characteristics in microwave frequencies. BST thin films have been deposited on various substrate materials, such as sapphire, MgO and glass, and the characteristics of thin film capacitors fabricated using BST have been investigated. The evaluation of electrical properties on in-plane type BST capacitors with inter-digital electrodes (IDE) and coplanar waveguide (CPW) type devices is important; however, there have been few detailed analyses or reports thus far. We prepared IDE and CPW electrodes of various sizes on BST films on sapphire. The size and voltage dependence of the capacitance and filter characteristics was determined. In this work, BST epitaxial films were deposited on sapphire substrates by rf magnetron sputtering. In order to obtain higher tunability, IDE electrodes with fine patterns below 1 µm were fabricated by electron beam (EB) lithography. The bias voltage dependence of the capacitance of the IDEs was measured. The tunability was found to increase with decreasing electrode spacing. The maximum tunability of 13.6 % was obtained at 100 nm IDE. CPW resonators of 6 mm length and 0.14 mm width were also prepared on BST (BST CPW) by EB lithography. The resonance frequency of 6.9 GHz of the BST CPW was calculated by a microwave simulator, which agreed with the measurement result of 7.47 GHz. A frequency shift of 1.1 % with bias voltage (50 V) was measured, revealing that a CPW type microwave tunable filter was successfully obtained on the BST/sapphire structure.