Development of a period-by-period EUV multilayer milling system for the final nm figure error correction by 0.1 nm per period

Abstract

A period-by-period ion milling system has been developed for final reflection wavefront error correction of an imaging EUV multilayer mirror by a stepwise 0.1 nm-per-period correction of the residual nm figure errors. For effective, gentle and uniform milling of the surface areas selected by a template, the system is designed with a rotating substrate holder exposed to a 150 mm-wide ion beam with a dose homogenizer mask plate. For demonstration of the wavefront correction principle, local milling of a dielectric multilayer mirror for visible light was carried out by the system. The wavefront as measured by a phase shifting interferometer showed that the reflection phase was "advanced" by the milling, which formed a geometrical depression at the multilayer mirror surface. This confirmed the physical optics principle of our method and proved the procedure of the method being promising and practical for the accurate reflection phase correction of an EUV multilayer mirror.