Characterization of the Photoacid Diffusion Length and Reaction Kinetics in EUV Photoresists with IR Spectroscopy

A soft-contact film transfer method was developed to prepare multilayer photoresist thin films that enable high-resolution spectroscopic and reflectivity measurements for determining the reaction−diffusion kinetic parameters and photoacid diffusion length. Infrared reflectance absorption spectroscopy was applied to follow, quantitatively, the reaction−diffusion kinetics during the post-exposure bake (PEB) step; the time evolution of the average deprotection level across a bilayer film with model photoresists is described by a kinetics model with three parameters: a reaction rate constant (kP), the phenomenological photoacid trapping constant (kT), and the photoacid diffusion constant (DH). A polymeric and molecular resist for next-generation extreme ultraviolet (EUV) lithography with chemically analogous structure was studied with this methodology. The three kinetic parameters follow an Arrhenius dependence but show quantitative differences between these two photoresists at a given PEB temperature. Further it was demonstrated that the photoacid diffusion length is not a simple function of diffusion coefficient; instead, it is dictated by all three kinetics parameters jointly in addition to the deprotection level at which the resist becomes soluble in an aqueous developer solution. These observations qualitatively explain the experimentally observed shorter photoacid diffusion length of the molecular resist in comparison to its polymeric counterpart.