EUV interferometry of the 0.3 NA MET optic

A new generation of 0.3 numerical aperture prototype EUV optical systems is now being produced to provide ail opportunity for early learning at 20-nm feature size. Achieving diffraction limited performance from these two-mirror, annular projection optics poses a challenge for every aspect of the fabrication process, including final alignment and interferometric qualification. A new phase-shifting point diffraction interferometer will be used at Lawrence Berkeley National Laboratory for the measurement and alignment of the MET optic at EUV wavelengths. Using the previous generation of prototype EUV optical systems developed for lithography research, with numerical apertures up to 0.1, EUV interferometers have demonstrated RMS accuracy levels in the 40-70 pm range. Relative to the previous generation of prototype EUV optics. the threefold increase to 0.3 NA in the image-side numerical aperture presents several challenges for the extension of ultra-high-accuracy.