In the past 30 years, feature sizes on IC's have dramatically decreased from 10 microns in 1975 to 0.18 microns in present IC production. This has been realized by increasingly complex optics, with higher numerical apertures and shorter wavelengths. State-of-the art lithographic tools now have numerical apartures up to 0.7 and use 193 nm deep-UV. As an ultimate extension of conventional optical lithography, 157 nm is now under development. For shorter wavelengths, no suitable lens materials are known and the arena is left to 'next-generation-lithography'. One propostion is extreme-UV lithography, employing multilayer mirrors around 13 nm in a vacuum system. In a research project at the Philips research Labs, we have developed a laser plasma source to generate EUV by focussing the laser into microscopic water droplets. The source has been coupled to a shearing interferometer employing transmission gratings carried by a 200 nm thin silicon membrane. Interferograms of a coated spherical substrate were recorded at the operational wavelength: 13 nm. The interferograms reveal the wavefront deviations with nm accuracy.