Optical properties of variable-composition aluminum/alumina thin films, part 1

Optical properties of alumina/aluminum thin films with varying compositions deposited by reactive sputtering have been studied. For measurements, ellipsometry has primarily been used, with additional measurements to provide more information. For modeling, the effective medium approximation has been used. This paper presents the ellipsometry and effective medium approximation approaches used. Results are presented for bare glass used as a substrate and for alumina films that are nearly stoichiometric. The microscope glass substrate was found to have much higher levels of absorption than quartz glass (SiO₂), for which the optical constants are commonly adopted for modeling. If this enhanced absorption had not been identified and quantified, it would have been inaccurately attributed to the deposited alumina films. Alumina films deposited at 5 to 9% oxygen flow ratios were also measured and modeled. Dispersion models (Sellmeier and Lorentz models) showed that, as the oxygen flow ratio decreased, the resonance energy also decreased. This suggests that adecreased oxygen flow ratio induces more defects, which situate themselves below the bandgap and eventually lower the bandgap. For these oxide films, a homogeneous composition assumption seemed satisfactory to describe the variety of optical measurements.