Shearing interferometer to characterize EUV optics with a laser plasma source

In order to assess the imaging quality of EUV optics, extensive metrology needs to be performed. Mirror substrates are commonly investigated with AFM, while optical probes and visible light interferometry are used to probe respectively high-, mid- and low spatial frequencies. Coatings on flat substrates are usually investigated using Cu-K(alpha) , i.e. X- ray reflectometry at grazing incidence. EUV-reflectometry can be performed on curved optics as well. Assembled EUV imaging systems have been evaluated by both visible-light and EUV interferometry. Most EUV interferometry has been performed with a point-diffraction interferometer, requiring the laser- like spatial coherence of a synchrotron. Early demonstrations indicated that the compact laser plasma source proposed for EUV lithographic tools could also be used. We have continued in this direction, and have demonstrated by means of computer simulation and experiments with visible light that an absolute accuracy of better than 10 milliwaves, i.e. 0.13 nm when using EUV, can be achieved by a shearing interferometer. This contribution describes the realization and first results of such an interferometer, employing a laser plasma as a EUV light source. The same scheme could however be used for all EUV sources proposed for EUV lithography.