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Progress along the path towards smaller semiconductor feature sizes continually presents new challenges. 157nm technology is a promising new step along this path. The major challenges encountered to date include environmental purging for high transmission and beam alignment in a purged environment at this short wavelength. We present a simple shearing interferometer consisting of two Ronchi phase gratings in series, used on axis. The common path set-up and zero optical path difference between the interfering diffraction orders makes this device both robust and easy to align. Ease of alignment is an added benefit when working remotely in a purged environment with low light levels. If one grating is shifted relative to the other, a phase shift is introduced and phase measurement techniques can be employed for high accuracy characterization of the incident wavefront. Set-ups, measurements and characterization of wavefronts and spatial-coherence at 157nm made with this device are presented.

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