Optical and crystalline inhomogeneity in evaporated zirconia films

Abstract

Electron-beam-deposited zirconia (zirconium dioxide) films are known to be inhomogeneous. They have a higher refractive index near the substrate and a lower index at the outer surface. Zirconia films deposited on high temperature (300°C) substrates are also known to be crystalline, exhibiting both a cubic and a monoclinic phase. X-ray diffraction studies of zirconia films of various thicknesses show that films which have an optical thickness of less than a quarter of a wavelength at 600 nm are cubic, while films that are thicker consist of both cubic and monoclinic zirconia. As the film thickness increases beyond a quarterwave optical thickness of 600 nm, the amount of the cubic phase remains constant and the amount of the monoclinic phase increases linearly with thickness. This implies that the film nucleates with a cubic structure but that after the film reaches a critical thickness, the surface conditions become more favorable for the growth of the monoclinic phase. This also suggests several possible models for the inhomogeneity in index. Spectral analysis indicates that the cubic portion of the film is optically inhomogeneous, while the monoclinic phase is homogeneous and has a lower refractive index.