High efficiency graded multilayer coating design using least-square fitting for NA0.55 extreme ultraviolet lithography Objective

The EUV (extreme ultraviolet) lithography system use a wavelength of 13.5 nm (nearly x-ray level), which will be absorbed by most materials. Therefore, in this paper, a high efficiency graded multilayer design method with least-square fitting approach was proposed to realize a high reflectivity multilayer coating for an anamorphic magnification EUV lithography objective with NA of 0.55. In this method, the parameters of the graded multilayer were calculated quickly by the least-square fitting method. Then calculate the reflectivity of each mirror separately. The design results show that the average reflectivity of each mirror is more than 60%, and the peak-valley value of reflectivity of each mirror is less than 4%.