Switchable and Tunable Ferroelectric Thin Film Radio Frequency Components.

Abstract

The future wireless communication systems demand frequency agility for multi frequency band operation and/or better spectrum utilization. Electronically reconfigurable transceivers are needed for such systems because of easier implementation, improved performance, and reduced size and cost. Multi-band multi-mode mobile phones and cognitive radios are two applications which will be benefited from reconfigurable transceivers. This thesis demonstrates novel switchable and tunable RF components using ferroelectric thin films. These components include tunable parallel plate capacitors using Barium Strontium Titanate (BST) thin films, tunable microwave bandpass filters using BST varactors, switchable BST thin film bulk acoustic wave resonators (FBAR), switchable Barium Titanate (BTO) contour mode acoustic wave resonators and switchable BTO FBAR ladder filters. A clean room fabrication process is developed to demonstrate the potential monolithic integration of ferroelectric thin films using the standard IC fabrication technology. Pulsed laser deposition is used to deposit ferroelectric thin films. The deposition conditions are optimized to achieve BST thin films with low loss tangent and good tunability. Ferroelectric thin films are characterized by a two port measurement technique at microwave frequencies. This technique provides a 30% uncertainty improvement in the loss and permittivity characterization compared to the commonly used one port measurement technique. For the first time, switchable acoustic wave resonators and filters using ferroelectric thin films based on the electrostriction are demonstrated. These resonators and filters can be effectively turned on and off by the application of DC bias voltage. Switchable BST FBARs are presented with quality factors exceeding 200 at 2 GHz. The electromechanical coupling coefficient is measured to be 7% which is comparable to piezoelectric aluminum nitride (AlN) resonators. A switchable BTO contour mode resonator is also demonstrated at 150 MHz. This enables monolithic integration of multiple frequency resonators and filters onto a single wafer. A switchable BTO FBAR ladder filter is demonstrated at 2.14 GHz with a 40 MHz bandwidth and an insertion loss of 6.2 dB and the out-of-band rejection of 20 dB. This filter has dimensions of 40 um by 80 um which are much smaller compared with AlN FBAR filters. Finally the future direction of this research is discussed.