Diffuser concepts for in-situ wavefront measurements of EUV projection optics

With the introduction of the NXE:3400B scanner, ASML has brought EUV to High-Volume Manufacturing for sub10nm node lithography. And work has already been started on a successor high-NA system with NA=0.55. For both these systems, node resolution will go down faster than NA increases, resulting in decreasing k1-factors and tightening of aberration requirements. A crucial component for measuring and controlling aberrations in-situ is a diffuser to fill the full pupil of the projection optics appropriately. This paper presents several new diffuser concepts, both reflective as well as transmissive, with their respective key performance metrics for both NA=0.33 and NA=0.55 EUV projection optics. These concepts can be used for measuring wavefront quality from dedicated fiducial plates, or for measuring directly from the imaging reticle. The latter would enable a combination of reticle alignment with lens aberration control without throughput penalty. It will be shown that with these diffuser concepts, we have a solution for in-situ aberration control for 5nm nodes and below.