High power durable SAW filter with epitaxial aluminium electrodes on 38.5° rotated Y-X LiTaO3 by two-step process sequence in titanium intermediate layer

High power durable electrodes with epitaxial aluminium (Al) films have been successfully grown on 38.5° rotated Y-X LiTaO₃ substrates. We reported epitaxial Al films on 64 rotated Y-X LiTaO₃ with titanium (Ti) intermediate layer in our previous papers. Despite a quite similar crystal structure, it has been difficult to obtain epitaxial Al on LiTaO₃ due to the different cut angle suitable for SAW devices. We found that a two-step sequence in the deposition temperature of Ti intermediate layer could make it possible for Al/Ti structure to grow epitaxially on 38.5° rotated Y-X LiTaO₃. It is most important for epitaxial growth to fabricate a Ti initial region, constructing the interface against the substrate, at high deposition temperature. Clear symmetrical spots were observed in x-ray pole figure from films prepared by a two-step process sequence, which suggests a twin structure of the Al crystal film. Power durability was evaluated from acceleration test as for ladder-type SAW filter with center frequency of 1.9GHz. Devices with epitaxial Al electrodes have had 3.3 × 10⁹ hours of estimated lifetime at input power of 0.8W and in ambient temperature of 50, which is drastically longer than those with polycrystalline electrode. Epitaxial electrodes with extremely less grain boundary can improve power durability because self-diffusion of Al atoms occurs mainly in the grain boundary of the film.