Hyper-NA EUV lithography: an imaging perspective

To print ever smaller features at high contrast projection lithography technology has evolved to shorter wavelength light and larger NA. After enabling the EUV wavelength, the industry is looking into increasing the NA. This study aims to identify EUV specific challenges regarding NA scaling beyond 0.55. We study if EUV imaging can still work at this higher NA and whether specific changes to the mask stack are required. At NA's much higher than 0.55, new effects like polarization will play a role, and larger impact of ultimate mask resolution and material interactions is expected. Already at NA 0.55, a small contrast loss is predicted due to the use of unpolarized light in the scanner. Further increasing the NA will enhance the contrast loss. We study these polarization effects in detail and assess their impact quantitatively for a set of generic building blocks. In addition, the larger incidence angles on mask when the NA increases above 0.55, will further enhance the M3D effects forcing additional mask changes. To enable proper imaging at high incidence angles, new mask architectures, that include changes in the EUV mask absorber and multilayer, will have to be tested using rigorous simulations.