Optical considerations of EUVL wavelength, NA, and multilayers at large angles

As EUV lithography is pursued at 13.5nm with numerical apertures (NA) of 0.55 and higher, the consequences of large angles on multilayer design, wavelength choice, and ultimate NA are considered. EUV multilayer designs for high-NA applications are presented. Reflectance amplitude and phase-shift analysis are carried out for Mo/Si multilayers up to angles associated with 0.55–0.75 NA EUV lithography, demonstrating the importance of considering phase effects. Through an understanding of the higher-order phase effects, defocus and spherical aberration can be attributed to the mask multilayer. Through RCWA lithographic image simulation, these effects can be confirmed. A depth-grading approach to optimizing multilayers is presented, utilizing the EUV optical properties of constituent materials to achieve improved performance by altering material ratios of the multilayer. By considering the polarization effects at high NA, both at the mask object plane and at the resist image plane, an alternative Ru/Be multilayer combination at a wavelength of 11.3 nm is considered. Through the introduction of an 11.3 nm/0.63 NA option for next-generation EUV lithography, the adverse polarization effects encountered at 13.5 nm/0.75 NA can be lessened, with additional improvement in focal depth. Design options are presented for Ru/Be depth-graded multilayers for optical coatings and mask applications. An evaluation is also presented for La/B multilayers for application at 6.6 nm as a potential EUV lithography option.