A transmissive spectral purity filter is configured to transmit extreme ultraviolet radiation. The spectral purity filter includes a filter part having a plurality of apertures to transmit extreme ultraviolet radiation and to suppress transmission of a second type of radiation. The apertures may be manufactured in carrier material such as silicon by an anisotropic etching process and topped with a reflective layer such as Mo metal, Ru metal, TiN or RuO. A diffusion barrier layer such as silicon nitride Si3N4, or silicon dioxide SiO2 is provided between the metal and the semiconductor to prevent diffusion and silicidation of the metal at elevated temperatures. The diffusion barrier layer may also serve as a hydrogen-resistant layer on parts of the semiconductor which are not beneath the reflective layer, and/or enhance emissivity for removal of heat from the structure.