Lithographic performances of aryl sulfonate ester-modified polystyrenes as nonchemically amplified resists†

Aryl sulfonate ester modified polystyrenes with different substituents (X–SEPS, X = H–, MeO–, and CN–) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The excellent thermal stability and film-forming capability of these three polymers suggest that they can satisfy the lithography process and are candidates for resist materials. Comparative electron beam lithography (EBL) demonstrates that the three resists (H–SEPS, MeO–SEPS and CN–SEPS) exhibit different EBL performances. Calculation of bond energies for the model compounds suggests that the influence of substituent groups on the bond energy is not the reason for the differences in sensitivity. Results obtained using a fully automated dissipative quartz crystal microbalance (QCM) analyzer confirm that the influence of substituent groups on the solubility behavior of resist films in developers leads to different photolithographic performances. The H–SEPS resist exhibits better comprehensive performance than the MeO–SEPS and CN–SEPS resists, achieving an 18 nm line/space (L/S) pattern and a 10 nm line/4 space (L/4S) semi-dense pattern by EBL at doses of 3200 and 2800 μC cm⁻², respectively. Further extreme ultraviolet lithography (EUVL) demonstrates the capability of H–SEPS resist to form 22 nm half-pitch (HP) patterns. The detailed study of the photochemical reaction and patterning mechanism suggests that the decomposition of sulfonate ester groups into polar sulfonic acid groups, along with a rearrangement, leads to a solubility switch of resist films in the developer.

Keywords: Nonchemically amplified resist; Reversible addition-fragmentation chain transfer polymerization; Aryl sulfonate; Electron beam lithography; Extreme ultraviolet lithography.