Dual nonionic photoacids synergistically enhanced photosensitivity for chemical amplified resists

In this paper, 4,5-dimethoxy-2-nitrobenzyl methacrylate (MONMA) was copolymerized with 2-hydroxyethyl methacrylate (HEMA) and tert-butyl methacrylate (TBMA) to obtain the terpolymer with dual sensitivity of ultraviolet (UV) light and acid, P(MONMA-HEMA-TBMA). The nonionic photosensitive ester, 4,5-dimethoxy-2-nitrobenzyl p-toluene sulfonate (MONS), is designed and synthesized as the prime photoacid generator (PAG) because of excellent photoacid generating capability of MONS and the similar substituted structure between MONS and MONMA. The ester groups in MONS PAG small molecules and the MONMA units of P(MONMA-HEMA-TBMA) disintegrate under UV irradiation to generate free diffused sulfonic acids and localized carboxylic acids, respectively. Both free and localized acids facilitate the subsequent acid-catalyzed hydrolysis of TBMA adjacent to MONMA units to induce exponential growth of carboxylic acids. Therefore, the hydrolyzed polymers containing high-content carboxylic acids become soluble in the alkaline developer. Hydrophilic HEMA is introduced to the photoresist resin to improve the interfacial affinity between the resist and the substrate. Dual synergy effect from free photoacid generator MONS and polymerized photoacid unit MONMA enhances photoacid generation and accelerates acid proliferation, which enables the decreased exposure dose and lower content of free photoacid generator MONS. A series of novel chemical amplification photoresists with various methacrylate monomer ratios are designed and synthesized to optimize the photolithographic effect of photoresist resin. The UV photolithography results demonstrate that the distinct exposed patterns are obtained at a low exposure dose using the photoresist formulation with P(MONMA50-HEMA19-TBMA31) and MONS of 9:1 w/w.