Synthesis of chemically amplified photoresist polymer containing four (Meth)acrylate monomers via RAFT polymerization and its application for KrF lithography

KrF photoresist polymers (PASTMs) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Four (meth)acrylates with lithographic functionalities including styrene (St), 4-acetoxystyrene (AST), 2-methyl-2-adamantyl methacrylate (MAMA), and tert-butyl acrylate(TBA) were used as monomer components and 2-methyl-2-[(dodecylsulfanylthiocarbonyl) sulfanyl]propanoic acid (MDFC) was used as RAFT agent, varying the RAFT content could modulate molecular weight. Fourier-transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (¹H NMR) indicated that the synthesis was successful. Gel permeation chromatography (GPC) showed that the molecular weight decreased with the increased content of MDFC, and all the polymers possessed weight-average molecular weight below ten thousand and polydispersity less than 1.32. Thermogravimetric analysis (TGA) characterized the thermal properties, the results implied that initial thermal decomposition temperature reached 200 °C, which could satisfy the lithography process. Differential scanning calorimetry (DSC) showed that the Tg decreases with molecular weight. The RAFT polymerization kinetics plots demonstrated that the polymerization was first-order, the number-average molecular weights of the polymers with relatively low polydispersity index values increased with total monomer conversions indicating that the concentration of growing radicals was constant throughout the polymerization process. The narrow molecular weight distribution and composition uniformity of the polymers prepared by RAFT polymerization could be beneficial for lithography, after alcoholysis, lithography evaluation under KrF lithography showed that this homogeneous polymer photoresist exhibited better space and line (S/L) pattern with resolution of 0.18 μm according to the SEM image.