Tunable materials and their microwave characterization

Tunable materials are those whose dielectric constant is dependent on a dc bias electric field. This property is a primary hallmark of ferroelectrics. Because of the loss issues, instead of in the ferroelectric state, this property is exploited in their paraelectric phase eventhough the tunability is slightly lower. Apart from these materials there are other oxides and biopolymers which are known to exhibit this property. This property can be exploited with small applied voltages if the material is in thin film form. A transmission line patterned on such a film can exhibit voltage dependent electrical length and this property can be manipulated to create a range of microwave devices like tunable filters, phase shifters, delay lines, attenuators etc. Such voltage dependent devices can easily be controlled by digital to analog converters (DACs) and hence by software. These devices find end applications in areas like phased array antenna, software defined radio and tunable metamaterials. The major issues in exploitation of the property exhibited by some materials are developing device quality thin films and extracting their microwave dielectric parameters like broadband dielectric constant, loss and tunability. New microwave characterization techniques are needed for thin films, taking in to account the fact that they are always deposited on a dielectric or conducting substrate and the thickness of the film is too small compared to the wavelength involved. Tunability measurement brings in additional issues like incorporating the dc field applying procedure in such a manner that it will not interfere with the microwave characteristics of the films or measuring instruments. In this paper we are demonstrating various techniques that can be used for the microwave characterization of tunable thin films. The microwave dielectric properties of thin films were characterized at different frequencies using different techniques by involving coplanar waveguide transmission lines (CPW) for broadband measurements and circular patch capacitor based measurement for measurement in the lower microwave bands.