Structure–Sensitivity Relationship of Base Self-Amplifying Polymers in 365-nm Photolithography: Experimental and Theoretical Studies

To improve the sensitivity of i-line photoresists, we investigated the structure–sensitivity relationship of six base self-amplifying photoresists. In this work, six base self-amplifying polymers with different core structures (piperidine, pyrrolidine, and morpholine) and substitution positions were synthesized. Their degradation properties were studied through thermal decomposition, solution decomposition, and evaluation of photoresist sensitivity. The results indicated that the P(3-Pyrrolidine) exhibited the best sensitivity, with an E0 of 7.9 mJ/cm2 (γ = 10.2) at a postexposure bake (PEB) temperature of 100 °C for 3 min. A sharp increase in sensitivity was observed at higher PEB temperatures or longer PEB times. The study revealed the following key findings regarding the structure–sensitivity relationship: 1) basicity, which accelerates hydrogen abstraction by lowering the activation barrier; 2) hydrogen bonding, where a strategically placed H-bond kinetically traps the system and inhibits the reaction; and 3) steric hindrance, which physically blocks reactant approach. This trend held true for all derivatives, including both the 2-substituted and the 3- or 4-substituted series, where stronger basicity resulted in higher sensitivity. Density functional theory (DFT) studies revealed a novel six-membered ring elimination mechanism for the Fmoc deprotection reaction, and the energy barriers for each step were also determined. We computed the energy barriers for monomers with different backbones and substituents. Based on these results, the rate-determining step of the reaction was identified, and weaker basicity of the parent core and greater steric hindrance led to a higher energy barrier, which in turn corresponded to lower photoresist sensitivity.