Tunability and Calculation of the Dielectric Constant of Capacitor Structures with Interdigital Electrodes

The voltage dependence of the dielectric constant of ferroelectric materials makes them attractive for use as tuning elements in microwave circuits. In this study, capacitance tuning and loss measurements were performed on ferroelectric materials prepared by the sol-gel process and RF magnetron sputtering. We find that Pb(Zr,Ti)O₃ (PZT) thin films with interdigital electrodes can be fabricated with reasonably low loss to make them useful for room temperature tuning applications. In addition, it is found that high temperature post-deposition annealing of both sputtered SrTiO₃ (ST) and sol-gel derived BaTiO₃ (BT) films markedly improves their tuning and loss factor characteristics. By annealing the samples in the range of 1000-1100°C, the tunability was increased by as much as a factor of seven, while the dissipation factors were decreased to values of 0.3-0.5%. In addition, it is shown that the permittivity of the films in these interdigitated capacitor structures can be calculated using an analytical model previously described by Farnell et al. [1].