Effects of crosslinking agent on lithographic performance of negative-tone resists based on poly(p-hydroxystyrene)

The effects of crosslinking agent on the lithographic performance of negative-tone resists based on poly(p- hydroxystyrene) (PHOST) have been investigated. Two crosslinking agents were used in the model resist systems: tetramethoxymethyl glycouril and 2,6-bis(acetoxymethyl) p- cresol. The resist containing tetramethoxymethyl glycouril shows better lithographic performance than the one with 2,6- bis(acetoxymethyl) p-cresol including better resolution, higher sensitivity, larger dissolution contrast, and less resist loss in the exposed regions during development. To reveal the origin of the difference in the lithographic performance, 13C NMR, FT-IR and differential scanning calorimetry (DSC) were employed to elucidate the crosslinking mechanisms, thermally induced crosslinking, the resulting network structure, and the crosslinking kinetics of the PHOST reaction with the two crosslinking agents. 13C NMR studies of model compound reactions suggest that the crosslinking of PHOST by tetramethoxymethyl glycouril proceeds via O- alkylation of the phenolic ring in the polymer, whereas the crosslinking of PHOST by 2,6-bis(acetoxymethyl) p-cresol proceeds via C-alkylation at the ortho carbon position of the phenolic ring. FT-IR studies indicate that there exists significant thermally induced crosslinking in the resist using 2,6-bis(acetoxymethyl) p-cresol as the crosslinking agent, while there is no detectable thermal crosslinking in the resist with tetramethoxymethyl glycouril as the crosslinking agent. The FT-IR results also show that the extent of crosslinking depends heavily on the crosslinking agent loading and the processing conditions. The effects of crosslinking chemistry on the crosslinked network structure and the lithographic performance of the two resists are discussed.