Characterization of an EUV Schwarzschild objective using phase-shifting point diffraction interferometry

We report wavefront measurement of a multilayer-coated, reflective optical system at 13.4-nm wavelength performed using a novel phase-shifting point-diffraction interferometer. Successful interferometric measurements of a 10x Schwarzschild objective designed for extreme ultraviolet projection lithography with 0.1-mu m resolution demonstrate high-precision with subnanometer resolution. The goal of the interferometry is to achieve wavefront measurement accuracy beyond lambda/50 rms at EW wavelengths. Preliminary measurements are discussed and the paths coward achieving the target accuracy are identified.