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The Solar Upper Transition Region Imager (SUTRI) was proposed to observe for the first time the Ne VII line at 46.5nm emitted from the upper solar transition region. As the key optical elements of the SUTRI, Sc/Si multilayer reflective mirrors are developed to offer high spectral selectivity and high reflectivity at 46.5nm with a normal-incidence angle. To avoid spectral contamination, the reflective bandwidth of the multilayer is required less than 4nm, which is achieved when the Sc layer thickness ratio is tuned to above 0.65. Meanwhile, the mechanical property, resistivity to thermal cycling, and temporal stability of the Sc/Si multilayer are characterized. The addition of an ultrathin Si layer (thickness of 0.6nm) in each Sc layer decreased the crystallization of Sc and flatted the interface, resulting in the enhancement of the mechanical property of the Sc/Si multilayer with new structure. After three times of thermal cycling, the temperature from 5°C to 40°C, the surface morphology of the new Sc/Si multilayer remained unchanged. The grazing incidence reflectometer test results showed that the periodic structure and thickness of the new multilayer were still similar after 2 years of storage. The optimized Sc/Si multilayer has a d-spacing of 24.55nm and a Sc thickness ratio of 0.72, achieving the extreme ultraviolet reflectivity of 28% at 46.1nm obtained from the laboratory-based reflectometer. The optimized Sc/Si multilayer mirrors have been applied in the SUTRI and received bright solar images at 46.5nm.
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