A new class of low bake resists for 193-nm immersion lithography

We report here, new non-acetal containing low bake (PEB < 100 degrees C) resists that are suitable for immersion lithography. These resists are based on novel low activation energy (low-E,) tertiary ester protecting groups. One major obstacle to imaging in the sub-50 nm regime using chemically amplified resists is the diminished image integrity in the pattern ("image blur") due to photo-generated acid diffusion into unexposed regions. Low processing temperatures are predicted to decrease the degree of photoacid diffusion and, in turn, decrease the image blur. Even though many low bake resist compositions have previously been reported, they are all based on acetal/ketal protecting groups. Unfortunately, these materials require a stoichiometric amount of water for the photoacid-catalyzed deprotection reaction to proceed. It is usually assumed that the water for the reaction comes from the environment in the bake station. However, fluctuations in humidity could affect the performance of the resist. Furthermore, acetal/ketal-based resists generally lack storage stability. For these reasons, acetal/ketal-based resists did not receive widespread acceptance in the lithography community. With the introduction of water based immersion lithography, acetal/ketal-based resists are expected to have further performance difficulties. Therefore, we targeted the development of new "low blur" resists for 193nm lithography that do not contain acetal/ketal protecting groups.