Challenges in high NA, polarization, and photoresists

Optical lithography is being pushed into a regime of extreme-numerical aperture (extreme-NA). The implications of the nonscalar effects of high -NA lithography (above 0.50) have been discussed now for many years1. This paper considers the consequences of imaging at numerical apertures above 0.70 with the oblique imaging angles required for low k1 lithography. A new scaling factor, kNA, is introduced to capture the impact of low k1 imaging combined with extreme- NA optics. Extreme-imaging is defined as k 1 and kNA values approach 0.25. Polarization effects combined with resist requirements for extreme-NA are addressed, especially as they relate to 157nm lithography. As these technologies are pursued, careful consideration of optical and resist parameters is needed. Conventional targets for resist index, absorption, diffusion, and reflectivity based on normal incidence imaging may not lead to optimum performance without these considerations. Additionally, methods of local and semi-local mask polarization are discussed using concepts of wire-grid polarizer arrays. Back-side and image-side polarization OPC methods are introduced.