Polyimides are an engineering plastic with excellent heat resistance, chemical resistance, and mechanical properties, and are used in the various industrial fields such as aerospace, automobile, display, and semiconductor. When used as dielectric layer for semiconductors, patterns are formed and applied for passage of wires. Since photosensitive polyimides (PSPIs) can form patterns directly in polyimide layer, many studies have been conducted. PSPIs are imidized after forming a pattern in the form of a polyimide precursor because of the insolubility of polyimide. Poly(amic acid) salt (PAAS), one of the polyimide precursors, has the advantage of good hydrolysis stability, but has the disadvantage of multi-stage polymerization through a polar organic solvent. As a solution to this, there is a method of polymerizing PAAS in water using an organic base as a catalyst. However, water-based polymerization has a problem in that functionalization is difficult because monomers and organic bases are limited.
In this study, photosensitive PAAS (PS-PAAS), a precursor of photosensitive polyimide, was synthesized and micro-patterns were implemented through water-based process. PS-PAAS was synthesized using water as a solvent, 3,3'',4,4''-Biphenyltetracarboxylic dianhydride (BPDA) acid dianhydride and p-phenylenediamine (pPDA) diamine as monomers. In this research, N-[3-(Dimethylamino)propyl]methacrylamide (DMAPMAA), 2-(Dimethylamino)ethyl methacrylate (DMAEMA), and 2-(Diethylamino)ethyl methacrylate (DEAEMA) were used as organic bases. For negative PS-PAAS composition, 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure2959) (Photoinitiator) and DL-Dithiothreitol (DTT) (Crosslinker) were added to PAAS-DMAPMAA, which has the highest solubility and molecular weight among the three organic bases. After spin coating this composition on a silicon wafer, a polyimide film was obtained through photolithography with 4000 mJ cm-2 UV exposure and stepwise thermal imidization up to 400 °C. As a result, a 92 nm thick patterned polyimide film having a resolution of 40-100 μm could be prepared. Finally, dielectric properties and device drive possibility were confirmed by fabricating an organic thin film transistor (OTFT).