Smoothing of diamond-turned substrates for extreme ultraviolet illuminators

Abstract

Condenser optics in extreme ultraviolet lithography (EUVL) systems are subjected to frequent replacement as they are positioned close to the illumination source, where increased heating and contamination occur. In the case of aspherical condenser elements made by optical figuring/finishing, their replacement can be very expensive (several hundred thousand dollars). One approach to this problem would be to manufacture inexpensive illuminator optics that meet all required specifications and could be replaced at no substantial cost. Diamond-turned metal substrates are a factor of 100 less expensive than conventional aspherical substrates but have insufficient finish, leading to unacceptably low EUV reflectance after multilayer coating. We show that by applying a smoothing film prior to multilayer coating, the high-spatial-frequency roughness of a diamond-turned metal substrate is reduced from 1.76 to 0.27 nm root mean square (rms), while the figure slope error is maintained at acceptable levels. Metrology tests performed at various stages of the fabrication of the element demonstrate that it satisfies all critical figure and finish specifications as an illuminator. Initial experimental results on the stability and performance of the optic in a real EUVL plasma source environment show no accelerated degradation when compared to conventional substrates.