Extreme ultraviolet free-standing transmittance filters for high brilliance sources, based on Nb/Zr and Zr/Nb thin films on Si3N4 membranes: Design, fabrication, optical and structural characterization

•Nb/Zr free-standing filter shows a peak transmittance of 60% at 7.02 nm.•Over 20 nm, the ratio of the out-of-band to the band transmittance goes lower than 2%.•Zr and Zr/Nb thin films deposited on Si3N4 techniques showed compressive stress.•Nb and Nb/Zr thin films deposited on Si3N4 showed tensile stress on the surface.•Nb/Zr filter shows better mechanical stability than Zr filter. Optical and structural properties of Niobium and Zirconium bilayer structures (Nb/Zr and Zr/Nb) were investigated in order to develop free-standing transmittance filters in the Extreme Ultraviolet region (EUV) between 5 and 20 nm. Samples of Nb/Zr and Zr/Nb were deposited on Silicon Nitride (Si3N4) membranes by magnetron sputtering technique, using metallic targets of Nb and Zr. A single layer of Zr and Nb on Si3N4 membrane has also been deposited and studied for a better understanding of the performance of these structures and their optical and mechanical properties. Optical microscope images of Zr and Zr/Nb structures on the membranes reveal compressive stress while Nb and Nb/Zr structures present tensile stress behavior. Nb and Nb/Zr self-standing filters were obtained by etching the silicon nitride membrane, with free-standing areas up to 3 × 3 mm2 with 100 nm of thickness. The transmittance performance of the samples has been measured by using EUV synchrotron radiation. The results show the highest peak transmittance of 60% at 7.02 nm and very good performance in the targeted range.