Effect of MgF2 deposition temperature on Al mirrors in vacuum ultraviolet

High reflectivity of mirrors is very important for many applications in the vacuum ultraviolet, such as for space observation, synchrotron radiation. This paper focuses on the substrate temperature's effect on the performance of Al mirrors when depositing the upper MgF2 layer. Al films are deposited on the substrates at room temperature by thermal evaporation, and a 5 nm MgF2 film is deposited on Al coating at room temperature immediately. Heating the substrate to various temperatures ranging from room temperature to 350 degrees C, then a 20 nm MgF2 film is deposited on the surface of Al/MgF2. The thickness of each layer is characterized using grazing incidence x-ray reflectivity. The reflectivity of sample is measured at the incident angle of 5 degrees in the wavelength range of 105 similar to 130 nm. The reflectivity of all samples fabricated at above room temperature is higher than the sample at room temperature below 115nm. The reflectivity of mirror at 350 degrees C temperature is lower than other mirrors, and the reflectivity of the samples at 300 degrees C and 200 degrees C is similar. There are more black dots on the surface of mirror at 350 degrees C than 300 degrees C, and no black dot on the surface of mirror at 200 degrees C. The measured results using surface profiler show that the black dots are small holes that increase the roughness of mirror and reduce the reflectivity. So the best temperature for depositing the upper MgF2 layer is in 200 similar to 300 degrees C to obtain high reflectivity of Al mirrors in vacuum ultraviolet.