Novel Photobase Generators Derived from Proazaphosphatrane–Aryl Borate for High-Pressure Mercury Lamp Lithography

Herein, novel borate-type photobase generators (PBGs) that generate a proazaphosphatrane known as Verkade's base (2,8,9-triisobuthyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (TTP)) were developed for photopatterning using a high-pressure mercury lamp. Eight PBGs were synthesized, each featuring distinct borate substituents: phenyl, p-fluorophenyl, p-tolyl, p-methoxyphenyl, m-fluoro-p-methylphenyl, 4-biphenyl, 2-naphthyl, and 6-methoxy-2-naphthyl. The relation between these substituents and their light absorption characteristics was analyzed via ultraviolet–visible spectroscopy and time-dependent density functional theory calculations. The PBG with the 6-methoxy-2-naphthylborate group (TTP–tetrakis[2-(6-methoxynaphthyl)]borate (TTP–TMNB)) demonstrated substantial light absorption near 365 nm (the i-line of the high-pressure mercury lamp), generating approximately 60 % of the corresponding TTP in deuterated tetrahydrofuran (THF) upon exposure to 650 mJ/cm², as confirmed via ¹H nuclear magnetic resonance spectroscopy. To evaluate its photolithographic potential, TTP–TMNB was mixed with epoxy resin (jER1001) and a poly(methacrylic acid)-co-poly(methyl methacrylate) copolymer (PMA_1.6-co-PMMA₁₀₀) in THF. The resulting mixture was spin-coated onto a silicon wafer and irradiated with the i-line through a photomask to create a negative-tone image. The sensitivity and contrast of this photopolymer system were measured to be 45 mJ/cm² and 1.0, respectively; moreover, a clear 10-μm negative-tone resolution was achieved.