Extreme ultraviolet alignment and testing of a four-mirror ring field extreme ultraviolet optical system

Extreme ultraviolet (EUV) interferometry has been used to characterize and align a recently fabricated, 4× reduction, four-mirror, aspheric optical system designed for EUV lithography. This system is called the Engineering Test Stand Set-1 Optic. An EUV phase-shifting point diffraction interferometer constructed on an undulator beamline at the Advanced Light Source was used to perform high-accuracy wavefront measurements during several alignment iterations. For each iteration, the alignment algorithm used 35 wavefront measurements recorded across the 26-mm-wide image-side ring field. Adjustments were made to systematically reduce the root mean square wavefront error magnitude to approximately 1 nm, bringing the system to nearly diffraction-limited performance.