Optimization of the reflectivity of magnetron sputter deposited silver films

Abstract

Silver films were deposited using direct current (dc) and midfrequency (MF) magnetron sputter deposition processes on floatglass in order to achieve maximum reflectivity over the entire visual and infrared spectral range. The films were investigated by means of ex situ and in situ spectroscopic ellipsometry, spectral photomety, electrical conductivity measurements, and atomic force microscopy. The following deposition parameters were varied: midfrequency and dc technique, power density, sputtering pressure, substrate temperature, sputtering gas (Ar, Kr, Ne), oxygen residual gas, and film thickness. With the aid of in situ spectroscopic ellipsometry, it can be shown that for certain process parameters an optimum layer thickness exists for achieving a maximum reflectivity in the visual spectral range. With increasing thickness, optical losses which are due to the formation of larger grains come into play. This optimum layer thickness is smaller for the films deposited by MF sputtering, indicating a smoother surface and smaller grain sizes compared to that of the dc mode. As a maximum value, a reflectivity of R=99.3% is achieved, which is close to the theoretical value, which was determined from the Drude fit parameters with respect to the resistivity. The resistivity of this film was found to be ρ=2.4 μΩ cm. The measured reflectivity of the sputter deposited films is comparable to e-beam evaporated silver films.