Physical mechanism of refractive index inhomogeneity of hafnium oxide thin film prepared by ion beam sputtering technique

The HfO₂ thin films prepared by ion beam sputtering are thinned after heat treatment. The optical constants of the thin films were obtained by inversion of the ellipsometric parameters. The crystal structure of the films was characterized by X-ray diffractometer. The results show that the correlation coefficient between the refractive index and the grain size is more than 90%. The refractive index increases with the increase of the grain size. The physical mechanism of the refractive index inhomogeneity in the film thickness direction is crystallization of thin films.