Characterization of the photoacid diffusion length

The photoacid diffusion length is a critical issue for extreme ultraviolet (EUV) lithography because it governs the critical dimension (CD), line-edge-roughness (LER), and line-width-roughness (LWR) of photoresist materials. Laboratorybased experimental methods that complement full lithographic testing would enable a rapid screening of materials and process conditions. This paper provides an approach to characterize the photoacid diffusion length by applying a bilayer stack technique. The method involves quantitative measurements of the deprotection kinetics as well as film thickness at each process step: radiation exposure, post-exposure bake, and development. Analogous to a contrast curve, by comparing the film thickness of the bilayer before and after development, the photoacid diffusion length was deduced in a commercial EUV photoresist and compared to EUV lithography. Further, by combining the experiments with kinetics modeling, the measured photoacid diffusion length was predicted. Lastly, based upon the measured kinetics parameters, a criterion was developed that next-generation resists must meet to achieve a 16 nm photoacid diffusion length. These guidelines are discussed in terms of correlations and contributions from the photoacid and resist properties. In particular, the trapping kinetics of the photoacid provides a route to reduce LER and the CD at low dose.