AT-WAVELENGTH INTERFEROMETRY FOR EXTREME-ULTRAVIOLET LITHOGRAPHY

A phase-shifting point diffraction interferometer is being developed f or at-wavelength testing of extreme ultraviolet lithographic optical s ystems. The interferometer was implemented to characterize the aberrat ions of a 10x Schwarzschild multilayer-coated reflective optical syste m at nm. Chromatic vignetting effects are observed and they demonstrat e the influence of multilayer coatings on the wave front. A subapertur e of the optic with a numerical aperture of 0.07 was measured as havin g 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 measuremen t repeatability of +/-0.008 wave (+/-0.11 nm) rms. Image calculations that include the effects of the measured aberrations are consistent wi th imaging performed with the 10x Schwarzschild optic on an extreme ul traviolet exposure tool. (C) 1997 American Vacuum Society.