Improving the optical and mechanical characteristics of aluminum thin-film filters by adding thin cap layers

Highlights: • Improving properties of thin film Al extreme ultraviolet filters • MoSi₂ and AlN protective cap layers enhance the filter's temporal stability. • Thin cap layers can lead to a significant increase in the film's strength. - Abstract: In this paper we investigated the influence of thin nanometer cap layers of aluminum nitride (AlN) and molybdenum disilicide (MoSi₂) on the temporal stability of transmission and the ultimate strength of aluminum (Al) film filters. An increase in the stability of the optical characteristics and in the mechanical strength of Al films both with AlN and MoSi₂ cap layers are shown in comparison with a monolayer Al film. The greatest efficiency was demonstrated in the case of Al with MoSi₂ cap layers. The obtained results show that the presence of MoSi₂ cap layers 2.5 nm thick on each side of an Al film significantly reduces (for example, by >4 times at the wavelength of 30.4 nm) the rate of film transmission loss in the extreme ultraviolet (EUV) range. In addition, the presence of cap layers can lead to a significant increase in the film's strength: the results of pressure bulge tests show that the Al with MoSi₂ cap layers film can withstand an ultimate load 1.5 times greater than an Al monolayer film of comparable thickness. The carried-out bulge tests indicate that an increase in the strength of the Al thin film in the presence of MoSi₂ cap layers may be due to the cap layers hindering crack initiation, which allows Al film to be plastically deformed. Acoustic tests of Al with MoSi₂ cap layers filters indicate the possibility of their use as entrance filters on a supporting mesh with a mesh size up to 10 mm in solar telescopes.