Extreme ultraviolet interferometric measurements of diffraction-limited optics

At-wavelength interferometric measurements of recently fabricated extreme ultraviolet (EUV) microstepper projection optics have revealed the highest performance for prototype EUV lithographic systems observed to date. The phase-shifting point diffraction interferometer is used to measure and align these two-mirror, multilayer-coated Schwarzschild optical systems designed with a numerical aperture of 0.088 and operating at 13.4 nm wavelength. Root-mean-square wave front error magnitudes as small as 0.60 nm have been achieved, actually exceeding the design tolerance set for these objectives.