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

铌酸钾KNbO_3 （KN）具有较大的压电常数，是实现宽频带表面声波（SAW）滤波器的理想材料。然而，由于发生了多次相变，很难生长出大的KN单晶。本研究的对象是在LiNbO_3 （LN）衬底上外延生长KN薄膜及其在宽带低损耗SAW滤波器中的应用。本课题研究了获得高取向KN薄膜所需的射频溅射条件、LN衬底外延面的形成以及KN/LN结构上的SAW性能。在以KN:K_2CO_3为混合靶材的单靶溅射中，随着衬底温度的降低，虽然K/Nb比增加，（220）面间距接近正交体KN，但取向性变差。利用单一富K靶K_5NbO_x，通过增加o_2自由基源的射频功率，获得了K/Nb比为0.40、（220）面间距接近正交体KN的高度（110）定向的KN薄膜。此外，原子力显微镜观察到沉积薄膜上的多晶粒结构。当电场作用于在取向KN薄膜上形成的数字间换能器时，观察到取向KN薄膜中的位移电流。这表明自发极化的存在。利用L_i_2O向外扩散技术，在y形切割的LN衬底上形成了一个平面间距接近正交体KN（200）平面间距的外延面。分析结果表明，在KN/LN结构的某些切口上，瑞利型SAW的耦合系数为35%，漏型SAW的衰减为零。

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.