Iridium/silicon capping layer for soft x-ray and extreme ultraviolet mirrors

Iridium terminated silicon spacer layers on Mo∕Si multilayer mirrors fabricated by dc magnetron sputtering are investigated for temporal stability. Samples maintained in atmosphere for periods of more than 3000 hours show: Ir terminating layers >16Å thick are stable, layers with thicknesses between ∼10 and 16 Å are not stable, and thicknesses between ∼7 and 10 Å are stable after a loss in reflectance of ∼1%. Extreme ultraviolet (EUV) reflectance, atomic force microscopy, sputter Auger electron spectroscopy, and x-ray diffraction measurements indicate that the terminating layer is, in reality, an alloy with graded composition that is Ir rich at the surface. The compositional gradient causes a degradation in reflectance from 66% for Si terminated multilayers to ∼63% for Ir terminated samples (thickness 18.7 Å). A sudden onset of oxidation induced silicon transport for deposited layers of Ir ∼10−16Å thick produces a degradation in EUV reflectance upon exposure to atmosphere. Accelerated lifetime testing of Si and Ir (18.7 Å) terminated multilayer mirrors show a ΔR∕R reflectance loss of 0.2% for the Ir terminated sample and ∼3% for the Si terminated sample. Further development on Ir terminated multilayer stacks to increase reflectance is needed.