Phase-measuring interferometry using extreme ultraviolet radiation

We describe initial experiments carried out with a phase-measuring, lateral-shearing interferometer that we have developed for use with radiation having a wavelength of about 13 nm. The system was developed for testing the imaging quality of extreme ultraviolet (EUV) lithographic cameras at the ,wavelength of their intended use. We describe here our initial work in which we measured essentially spherical wave fronts obtained by focusing undulator radiation onto a small pinhole. The interferometer is shown to have a sensitivity of 0.021 waves rms, or better, at an operating wavelength of 13 nm, which is more than sufficient to determine whether or not an EUV imaging system exhibits diffraction-limited performance. (C) 1995 American Vacuum Society.