Fabrication of extreme-ultraviolet point-diffraction interferometer aperture arrays

Interferometric testing at the design wavelength is required for accurately characterizing the wave front of an imaging system operating in the extreme ultraviolet. The fabrication of point-diffraction interferometer apertures for extreme ultraviolet wave-front aberration analysis is described. The apertures are formed in a 200-nm-thick low-pressure chemical-vapor-deposited Si(3)N(4) film and vary in size from approximately 0.10 to 0.50 µm to generate a reference wave front of varying numerical aperture. A graded absorber overcoat is used to control the intensity of the aberrated wave front.Optimal fringe contrast can be obtained when the aperture that provides the maximum uniformity and contrast in the interference plane is selected.