Effect of the annealing treatment on the physical and structural properties of LiNbO3 thin films deposited by radio-frequency sputtering at room temperature

Abstract

Reaching the full potential of the 5th Generation of mobile applications (5G) requires pushing the performances of acoustic radio-frequency (RF) filters beyond their current limits. Current high-performances RF filters (above 2.5 GHz) are based on aluminium nitride (AlN). Their bandwidth is however limited by the low electromechanical coupling coefficient of AlN. In this context, due to its higher electromechanical coupling coefficient, lithium niobate (LiNbO3) is a good candidate as an active piezoelectric material. For this application, LiNbO3 films need to be stoichiometric and to be either highly textured or monocrystalline. However, there seems to be no deposition process for high quality LiNbO3 thin films on silicon available at the moment, which could be mainly due to the difficulty to achieve a high crystalline quality and to the challenging control and of its stoichiometry. In the present work, the realization of highly textured films deposited directly on silicon substrate by RF sputtering at room temperature and after annealing was demonstrated. The effect of the annealing parameters (temperature and ramping rates) was studied. LiNbO3 layers with a strong degree of texture along the c-axis were obtained. The X-ray diffraction analysis showed that an optimum texture of 99% can be reached by a high temperature of annealing and a fast temperature ramping rate. The effect of annealing parameters on the presence of secondary phase LiNb3O8 and film density was also investigated.