Morphology of lithium niobium oxide thin film ultrasonic transducers deposited by RF magnetron sputtering

Abstract

Lithium niobium oxide (LNO) thin film high temperature ultrasonic transducers have been deposited using radio frequency magnetron sputtering. Effects of the sample position in the sample plane on the thin film morphology are studied. In the center of the sputtering area, where the radial distance is 0 mm, the LNO thin film transducers are a mixture of LiNbO3/LiNb3O8 phases with well developed columnar structure and good ultrasonic response. At the radial distance of 10–40 mm, complex “sunflower-like” morphological features are formed, which may develop due to preferable crystallization of the sputtered material on particle seeds, significant difference in atomic mass of Li and Nb, and space confinement. The formation of wedge-like “sunflowers” reduces the performance of the ultrasonic transducers. In addition, the impacts of annealing temperature and time on morphology, crystal structure, element distribution, and ultrasonic response are also observed. The long-term annealing at 700 °C (for 100 h) upgrades the ultrasonic response, and for prolonged annealing up to 312 h, the ultrasonic response gradually degrades.