Integration of Ferroelectric Thin Films in Tunable Microwave Devices

Abstract

Abstract

LAM, PETER GAIFUN. Integration of Ferroelectric Thin Films in Tunable Microwave Devices. (Under the direction of Jon-Paul Maria).

Recent growth in wireless communication industry has led to a growing search for alternative technologies that can provide higher flexibility and efficiency in the handling of radio frequency spectrum, and preferably at a similar or lower cost than the present technologies. Thin film barium strontium titanate has shown great promises for microwave applications. There are two goals in this thesis: 1) To show a methodology for integration of BST thin films in microwave devices operating at frequencies above 6 GHz, and 2) To explore the limits of material optimization for increased device performance. The integration of BST thin film with a 3rd order combline-based bandpass filter in the frequency range of 6 to 18 GHz was demonstrated. Alumina substrates with filled vias were used to support the devices. Optimized (Ba0.7Sr0.3)TiO3 film was deposited and patterned in alumina substrate. Planar gap capacitors were patterned with a Cr/Au metal stack while the rest of the device layout was patterned with silver metal and electroplated with copper. The measured microwave characteristics of the bandpass filters fall within the specifications of the design but more improvements were needed to lower the insertion loss and the frequency tunability for devices operating at frequency above 12 GHz. The metallization impact on the microwave devices was studied. Two variables were tested with electroplated copper: 1) Thickness, and 2) geometry with respect to the gap. Increasing the thickness of the copper decreases the insertion loss of the device. Metal thickness beyond three skin depths does not impart significant improvement. Plating distances to the edge of the gap in the capacitor has not effect in the insertion loss of the filter. Tunability limit of coplanar gap capacitors was investigated. A series of gap capacitors with different geometry were tested. In the limit of low capacitance values, a fringe capacitance is observed, which measured to be in the range of 50-100 fF. The non-tunable capacitance contributes to the low tunability of the 12-18 GHz range bandpass filter, where the required capacitance values for impedance matching, 200-400 fF, are close to the values of the fringe. A coplanar MIM capacitor structure is proposed to improve the tunability values. The configuration consists of a sapphire substrate, followed by the metal coplanar electrodes (Pt/Ti stack deposited by e-beam evaporation) and then capped with the BST layer. Tunability values of 50% were obtained with an electric field of 125 KV/cm for an interdigitated coplanar MIM capacitor.