EUV interferometry of a four-mirror ring-field EUV optical system

At-wavelength, extreme ultraviolet interferometric measurements of a new, four-mirror, ring-field projection optical system have been made. Designed for operation at 13.4-nm wavelength with a 0.1 numerical aperture and a 26 mm field of view at the wafer, the nearly diffraction-limited wavefront quality of the system has been verified interferometrically. After assembly and alignment with visible-light interferometry, the optic was transported to Lawrence Berkeley National Laboratory where the at-wavelength testing with a phase-shifting point diffraction interferometer was performed. Measurement of the system wavefront at a number of points across the field of view reveals the optical performance of the system over its large, ring-field imaging area.