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Tunable Microwave Devices

Thin films are very attractive for microwave tunable applications due to the low tuning voltages and the relatively low production cost. Microwave ferroelectric phase shifters are the most widely studied tunable ferroelectric components. Their importance stems from the role they could play in phased array antennas. A phased array antenna consists of thousands of radiating elements which should be served by thousands of phase shifters. The phase shifters are used to modify and control the width and angle of the steered radar beam. At present each phase shifter is a housed microwave semiconductor module. The use of ferroelectric films enables the integration of the phase shifters with the microwave circuits on one substrate thus substantially reducing the size, mass, and cost of the antennas. A simple coplanar line structure patterned on a ferroelectric thin film coated substrate makes a phase shifter in which the phase velocity of the electromagnetic wave that passes through the line is controlled by the applied dc electric field (via changing the permittivity of the film, which in turn controls the wave velocity). It is also possible to tune the phase velocity of the electromagnetic wave passed through the coplanar line by loading this line with tunable ferroelectric capacitors. Electrically tunable microwave devices offer considerable savings in weight, size, and ease of use over conventional, mechanical alternatives. Therefore they are of great importance in earth satellite rf communications. The para electric materials such as Ba0.5Sr0.5TiO3 and BZN (Bi1.5ZnNb1.5O7) shows tunability over the dielectric constant and consequently they show a high phase shift. The dielectric loss factor of these materials are very low. Tunable microwave devices are fabricated on the deposited thin films by lithography. Figure shows a tunable microwave phase shifter patterned on a ferroelectric thin film. Tunable microwave phase shifters are fabricated by patterning Inter Digital Capacitors (IDCs) on the ferroelectric thin film.

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