Influence of ex-situ annealing on the properties of MgF2 thin films deposited by electron beam evaporation

We report on the properties of magnesium fluoride (MgF2) thin films deposited by electron beam evaporation as a function of substrate deposition temperature and ex-situ annealing temperature. In particular, we report on the dependence of refractive index on annealing temperature, which can be used as a tuning parameter of the optical properties. Mechanical and structural properties of the films influenced by the annealing are also examined. Changing the substrate temperature from 50 °C to 240 °C caused a decrease of the refractive index and the lowest value of 1.36 (measured at 632.8 nm) was achieved for the substrate temperature of 240 °C. Rapid thermal annealing further decreased the refractive indices to slightly below 1.32. This could indicate increase in the film porousness and removal of adsorbed water molecules. Prior annealing the film surfaces were very smooth with root mean square and mean roughness below 1 nm. Annealing above 700 °C changed the structure of the films drastically, as they started to form a granular structure, while an annealing temperature of 1000 °C increased the refractive index to a value as high as 1.5. Using X-ray photoelectron spectroscopy we show that the surface of the films consist mainly of Mg and F atoms, but also small traces of C and O are present. The Mg:F ratio remained essentially the same (43:57) between different deposition temperatures. To demonstrate the need for post-deposition annealing treatment, we have also studied the aging effect in the MgF2 based anti-reflective coatings.