Integration of sol-gel frequency agile materials for tunable RF devices

• Main Author: Fragkiadakis, Charalampos
• Other Authors: Kirby, Paul B.
• Published: Cranfield University 2012
• Subjects: 621.381522
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.558558

This thesis focuses on the use of high permittivity tunable dielectrics and more specifically sol-gel ferroelectric thin films for low cost, high performance tunable devices such as varactors and filters at RF and microwave frequencies. The top- ics covered include measurement techniques for the characterization of tunable dielectrics at low and microwave frequencies, fabrication processes, electrical and acoustic modeling of thin film ferroelectric varactors, performance optimization using conductive electrodes, realization of tunable microwave circuits and inte- gration of tunable dielectrics with conventional bulk acoustic wave resonators (FBAR). A lead strontium titanate (PST) sol-gel ferroelectric varactor is designed, elec- trically and acoustically modeled and fabricated, displaying dielectric tunability of "'-'75%. A two port automatic extraction technique using MATLAB allowing the de-embedding of parasitic connecting transmission lines, as well as parasitic pads has been developed and presented, yielding accurate dielectric permittivity values in good agreement with literature. The potential factors that may compro- mise the electrical performance of the ferroelectric tunable varactor are analyzed and a novel Au/Ti02 bottom electrode stack process is proposed and shown to improve the RF performance of the tunable varactor lowering the overall metaliza- tion resistance and improving performance, compared to the commonly used Pt electrodes. To establish the possibility of tunable microwave systems integrating sol-gel ferroelectric tunable varactors the following novel microwave devices are designed, modeled and fabricated: A ferroelectric varactor-based RF resonant switch, integrating a thin film sol- gel PST ferroelectric varactor with a high Q micro-machined inductor is fabri- cated. An insertion loss of ",1.5 dB and isolation of ",18 dB have been achieved for a single 7 GHz resonant switch with a device area of 0.6 mm x 1 mm. The intrinsic performance limitations of this type of device due to the ferroelectric thin film are discussed and the implementation of cascaded switches and state-of-the- art ferroelectric materials for further improvement of performance of this device, have been considered and simulated. Tunable band-stop resonators and notch filters using sol-gel PST ferroelectric varactors in a coplanar waveguide (CPW) defected ground structure are fabricated and measured. The PST varactors tune single resonators and 3-pole band-stop filters, operating at the center frequency of 4 and 8 GHz, having a maximum rejection of more than 13.8 dB at the stop band, while the insertion loss at the pass band is less than 3 dB. Full-wave analysis is performed to identify the critical points, where PST varactors are implemented to adjust the resonance frequency of the devices. An optimized fabrication process allows for fabrication of a 3-stage filter with a maximum rejection of 28 dB, albeit with a reduced tuning range, possibly due to DC bias path leakage. Finally, a fabrication approach where a ferroelectric varactor is integrated with a conventional zinc oxide (ZnO) acoustic wave resonator is presented. The approach avoids the piezoelectric thin film degradation due to the ferroelectric annealing by first fabricating the ferroelectric varactor and superimposing the conventional FBAR on top of it. The tuning of the series resonant frequency of a conventional ZnO FBAR with a ferroelectric varactor is demonstrated. Field induced deformation limits the maximum shift of the resonance to 0.45% at 1.5 GHz, for 41% tunability of the ferroelectric varactor, suggesting a big scope for possible improvements in performance by improving the design and fabrication. VIII.