Characterization of dielectric columnar thin films by variable angle Mueller matrix and spectroscopic ellipsometry

Thin films composed of MgF2 and SiO2 were grown by physical vapor deposition, with the vapor flux arriving at oblique angles, ranging between α = 60° and 86° (with respect to the substrate normal) but with a focus on the glancing angle deposition (GLAD) regime where α > 80°. The resulting films are composed of columns with elliptical cross sections slanted towards the vapor source, and in the case of higher α, the columns are isolated from each other. These films were examined by variable angle Mueller matrix ellipsometry, and using the Bruggemann effective medium approximation (EMA). The films were found to be optically biaxial with the first principal index of refraction along the direction of the columns, the second also in the deposition plane but perpendicular to the first, and the third orthogonal to the deposition plane. Films composed of SiO2 were also found to exhibit an index gradient from the substrate to the surface. The indices of refraction for MgF2 and the 50% growth point of SiO2 were found to range from 1.096 to 1.38, and from 1.099 to 1.46 respectively, at a wavelength of 500 nm, and were found to decrease as the deposition angle increased. The film densities as compared with those of films grown at normal incidence were obtained optically and compared with previous simulation work. The film densities were found to be independent of the film thickness for MgF2 films ranging in thickness from 500 nm to 3400 nm for films grown at α ≅ 80°. The measured optical properties of these films provide a basis for a model of tilted columnar GLAD-grown thin films. This basic model can be extended to describe more complex GLAD film structures.