Composition-Dependent Lithographic Performance of 3-Arm Star Photoresists

The lithographic performance of chemically amplified resists is strongly influenced by the balance among the functional components of the resist polymer. In this study, a series of 3-arm star terpolymers containing 2-hydroxyethyl methacrylate (HEMA), tetrahydrodicyclopentadienyl methacrylate (TCDMA), and the acid-labile monomer 2-methyl-2-adamantyl methacrylate (MAMA) were synthesized via core-first reversible addition–fragmentation chain-transfer (RAFT) polymerization using a trifunctional TMP-CEPA3 chain-transfer agent. Three polymers, denoted HTM 442, HTM 334, and HTM 226, were prepared by systematically varying the MAMA fraction. 1H NMR analysis confirmed different incorporated monomer compositions, while GPC analysis showed relatively narrow molecular-weight distributions for all polymers. Thermal analysis revealed that the thermophysical properties were governed by the combined effects of HEMA, TCDMA, and MAMA rather than by the MAMA fraction alone. Negative-tone lithographic evaluation using 365 nm UV exposure and n-butyl acetate development demonstrated composition-dependent differences in pattern formation. Among the investigated compositions, HTM 334 exhibited the most favorable line-pattern morphology, with relatively uniform line profiles and lower LER values at most of the investigated nominal mask dimensions. HTM 442 also formed distinguishable patterns, whereas HTM 226 did not produce discernible patterns under the evaluated conditions. These results indicate that increasing the acid-labile MAMA fraction does not lead to a monotonic improvement in lithographic performance within the investigated composition range, highlighting the need to consider the overall copolymer composition when designing 3-arm star photoresists.