Characterization of polycrystalline AlN films using variable-angle spectroscopic ellipsometry

Abstract

Polycrystalline aluminum nitride (AlN) films, prepared by reactive sputtering, were characterized by variable-angle spectroscopic ellipsometry (VASE) and x-ray diffraction (XRD) rocking curve. A VASE model, which included cylindrical symmetry, effective-medium approximation, and surface roughness, was used to extract the optical constants of AlN films: refractive index (n) and extinction coefficient (k). Microstructure of the sputtered polycrystalline AlN films was well reflected in the VASE model and was verified by scanning electron microscopy cross-sectional micrographs. In addition, a correlation between optical constants and full width at half maximum (FWHM) of XRD rocking curve was established. Films with smaller FWHM of the XRD rocking curve, an indication of fewer defects and better crystallinity, had a higher refractive index and smaller extinction coefficient. This study leads to a faster and simpler optical method for characterizing sputtered AlN films compared to the currently predominately used XRD rocking curve method. Integration of VASE in a sputter deposition system to monitor films’ thickness and quality in situ would also be useful for production applications.