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Interplay Between Thin Film Ferroelectric Composition, Microstructure and Microwave Phase Shifter Performance

One of the keys to successfully incorporating ferroelectric films into Ku-band (12 - 18 GHz) phase shifters is to establish the composition, microstructure and thickness required to meet the tuning needs, and tailor the film properties to meet these needs. Optimal performance is obtained when the film composition and device design are such that the device performance is limited by odd mode dielectric losses, and these losses are minimized as much as possible while still maintaining adequate tunability. The parameters required to maintain device performance will vary slightly depending on composition, but we can conclude that the best tuning-to-loss figures of merit (K-factor) are obtained when there is minimal variation between the in-plane and out-of-plane lattice parameters, and the full-width half maximum values of the BSTO (002) peaks are less than approximately 0.04. degrees. We have observed that for phase shifters in which the ferroelectric crystalline quality and thickness are almost identical, higher losses are observed in films with higher Ba/Sr ratios. The best performance was observed in phase shifters with Ba:Sr = 30:70. The superiority of this composition was attributed to several interacting factors: the Ba:Sr ratio was such that the Curie temperature (180 K) was far removed from room temperature, the crystalline quality of the film was excellent, and there was virtually no difference between the in-plane and out-of-plane lattice parameters of the film.