SiNx thickness dependence of spectral properties and durability of protected-silver mirrors

Both long-term environmental durability and high broadband reflectance of protected-silver mirrors are of great importance to the developments of optical instruments for a variety of applications under harsh environments. In this paper, the dependences of spectral properties and environmental durability of protected-silver mirrors with a Sub/NiCrNx/Ag/NiCrNx/SiNx/Air structure on thickness of SiNx protective layer were investigated in details. The reflectance, transmittance, total scattering loss measurements and scanning electron microscope (SEM) imaging were employed to characterize the spectral properties and surface morphology, and accelerated environmental tests, including humidity test and salt fog test, were applied to investigate the environmental durability. Five magnetron-sputtering prepared protected-silver mirrors with SiNx layer thicknesses between 8 nm to 16 nm were tested. The results showed that both optical and corrosion-resistant properties of protected-silver mirrors were SiNx thickness dependent and the optimum SiNx thickness should be approximately 14 nm for Sub/NiCrNx/Ag/NiCrNx/SiNx/Air protected-silver mirrors to have both reasonably high reflectance in the visible spectral range and high corrosion resistance for long lifetime applications in harsh environments.