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

Abstract

AbstractHerein, 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 (PMA1.6‐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.