Shearing interferometry for at-wavelength wave front measurement of EUV projection optics

Summary form only given. We propose a simple and compact lateral shearing interferometric optical system for at-wavelength wave front measurement of the extreme-ultraviolet lithography projection lens. The proposed lateral shearing interferometer has a common path configuration, which essentially can be operated over a spatially incoherent EUV light sources, therefore it is feasible to use EUV plasma sources. The outgoing wave front from the projection lens is detected by the grating lateral shearing interferometer based on algorithms for phase-shifting fringe analysis to suppress the effect of higher order interference patterns. A total of 1e12 photons from the light source must be collected for testing a 6-mirror projection system at present. The required grating pitches can be designed in excess of 90 nanometers for < NA 0.3 projection optics, which are considered to be possibly fabricated. The effects of measurement error sources have been characterized and the measurement accuracy is estimated to be within 0.1 nm.