An at-wavelength interferometer is being created for the measurement and alignment of the 0.3 numerical aperture Micro Exposure Tool (MET) projection optic at extreme ultraviolet (EUV) wavelengths. The prototype MET system promises to provide early learning from EUV lithographic imaging down to 20 nm feature size. The threefold increase to 0.3 NA in the image-side numerical aperture presents several challenges for the extension of ultrahigh-accuracy interferometry, including pinhole fabrication and the calibration and removal of systematic error sources. (C) 2003 American Vacuum Society.