Microstructural and chemical analysis of polycrystalline LiNbO3 films obtained by room-temperature RF sputtering after various annealing durations

The development of materials for industrial applications is in constant evolution. Regarding environmental or health laws, the use of some materials is restricted (lead-based). In this context, lithium niobate (LiNbO3) is studied in order to develop the performances of the RF (radio frequency) applications (for instance, filters using the bulk acoustic wave or surface acoustic wave technology). The deposition of LiNbO3 is mainly performed by chemical or physical vapor deposition and solgel techniques, resulting in polycrystalline films with or without a preferred orientation. For RF applications, LiNbO3 films need to be highly textured with a preferred orientation among the (006) plane in order to maximize the electrical response. In the present work, 150 nm thick LiNbO3 films were grown by RF sputtering at room temperature on Si substrates with native SiO2. The films were annealed at 950 °C, with an original process, for various durations. Polycrystalline yet (006) preferentially oriented LiNbO3 films were obtained on silicon. The properties of the as-deposited and annealed films were analyzed by x-ray diffraction (XRD), Raman spectroscopy, secondary ion mass spectrometry (SIMS), and scanning electron microscopy (SEM). The annealing treatment effect was highlighted by symmetric XRD analysis. The adhesion of the films is confirmed by SEM analysis. The diffusion profile of Li was characterized by SIMS and XPS analyses. This process, mainly based on a specific annealing treatment, leads to the synthesis of nearly stoichiometric LiNbO3 on silicon, and the effect of the thermal treatment on the texture of the film is highlighted.