Normal incidence multilayer gratings for the extreme ultraviolet region: Experimental measurements and computational modeling

The normal incidence efficiencies of a 2400 groove/mm master grating and a replica grating were measured using synchrotron radiation in the extreme ultraviolet region. As a result of the replication process, the replica grating had an aluminum surface with an oxide layer. An additional thin SiO2 coating was applied to the oxidized aluminum surface for the purpose of reducing the microroughness. The efficiency of the replica grating as a function of wavelength had an oscillatory behavior that resulted from a thin-film interference effect associated with the SiO2 coating. The measured efficiencies were compared to the efficiencies calculated by a computer program that is based on the modified integral approach. The calculation accounts for the groove profile as determined from atomic force microscopy (AFM), the optical properties of the grating surface and the three layers (Al, Al2O3, and SiO2,)and the polarization of the incident radiation. The measured and calculated efficiencies are in good overall agreement.