At-wavelength interferometry for extreme ultraviolet lithography

A phase-shifting point diffraction interferometer is being developed for at-wavelength testing of extreme ultraviolet lithographic optical systems. The interferometer was implemented to characterize the aberrations of a 10x Schwarzschild multilayer-coated reflective optical system at nm. Chromatic vignetting effects are observed and they demonstrate the influence of multilayer coatings on the wave front. A subaperture of the optic with a numerical aperture of 0.07 was measured as having a wave front error of 0.090 wave (1.21 nm) root mean square (rms) at a 13.4 nm wavelength. The wave front measurements indicate measurement repeatability of +/-0.008 wave (+/-0.11 nm) rms. Image calculations that include the effects of the measured aberrations are consistent with imaging performed with the 10x Schwarzschild optic on an extreme ultraviolet exposure tool. (C) 1997 American Vacuum Society.