Fabrication of Highly Functional Potassium Niobate Piezoelectric Thin Films and Its Application to Wide-Band Low-Loss Surface Acoustic Wave Filters

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Potassium niobate KNbO_3 (KN) is a promising material for realizing wide-band surface acoustic wave (SAW) filters owing to its large piezoelectric constant. However, it is difficult to grow a large KN single crystal due to the occurrence of several phase transitions. The objects of this study are the epitaxial growth of KN films on LiNbO_3 (LN) substrates and its application to the wide-band and low-loss SAW filters. In this project, the RF-sputtering conditions necessary for obtaining a highly oriented KN thin film, the formation of the epitaxial plane on LN substrate, and the SAW properties on KN/LN structure were investigated.1. The KN thin films were deposited on MgO substrate using several types of target In single-target sputtering with a mixture target of KN:K_2CO_3, as the substrate temperature was decreased, although the K/Nb ratio increased and the (220)-plane spacing approached that of orthorhombic bulk KN, the orientation deteriorated. Using a single K-rich target, K_5NbO_x, a highly (110)-oriented KN thin film with a K/Nb ratio of 0.40 and a (220)-plane spacing near that of orthorhombic bulk KN was obtained by increasing the RF power applied to the O_2-radical source. Moreover, a multigrain structure on the deposited thin films was observed by atomic force microscopy. The displacement current in the oriented KN thin film was observed when an electric field was applied to the interdigital transducer formed on the thin film. This indicates the existence of spontaneous polarization.2. By utilizing L_i_2O out-diffusion technique, an epitaxial plane with a plane spacing close to the (200)-plane spacing of orthorhombic bulk KN was formed on Y-cut LN substrate.3. The analytical results showed that the coupling factor of 35% for a Rayleigh SAW and zero-attenuation for a leaky SAW are obtained on certain cuts of the KN/LN structure.