Recent advances of 3-aspherical mirror system for EUVL

A three-aspherical-mirror system for Extreme Ultraviolet Lithography has been developed. The aspherical mirrors were fabricated using the computer controlled optical surfacing (CCOS) process and a phase shift interferometer. The mirrors have a figure error of 0.58 nm and surface roughness of 0.3 nm. In order to obtain a high efficiency mirror, M1 and M2 were coated with a graded d-spacing Mo/Si multilayer and mirror M3 was coated with a uniform d-spacing Mo/Si multilayer. The peak reflectivity is 65% at the wavelength of 13.5 nm. The wavelength matching of each mirror spans 0.45 nm. The mirrors were aligned with a Fizeau-type phase shift interferometer, and a final wavefront error of less than 3 nm was achieved. Exposure experiments carried out at New Subaru synchrotron facility revealed that this system is capable of replicating a 56 nm pattern in a 10 mm×1 mm exposure field.