Phase-Shifting Point Diffraction Interferometry for At-Wavelength Testing of Lithographic Optics

We report on the development of at-wavelength wavefront metrology for evaluation of extreme ultraviolet optics intended for use in projection lithography. Initially, a point diffraction interferometer was used and the experience acquired with this instrument led to the development of a new interferometer, the phase-shifting point diffraction interferometer. In this paper, point diffraction interferometry performed at EUV wavelengths is discussed. The design and implementation of the new phase-shifting point diffraction interferometer are described.