Colorless Polyimides Derived from Cycloaliphatic Tetracarboxylic Dianhydrides with Controlled Steric Structures (4). Applications to Positive-type Photosensitive Polyimide Systems with Controlled Extents of Imidization.

Abstract

Novel colorless polyimides (PIs) derived from 1R, 2S, 4S, 5R-cyclohexane tetracarboxylic dianhydride (a new hydrogenated pyromellitic dianhydride (PMDA) with a controlled steric structure, H"-PMDA) with various aromatic diamines were applied to positive-type photosensitive PI systems. The H"-based PI precursors, poly(amic acid)s (PAAs), were partially imidized with controlled extents of imidization (χI) by adding established amounts of a cyclodehydration reagent (acetic anhydride/pyridine) into the PAA solutions and stirring at room temperature for 24 h while keeping the homogeneity of the reaction mixture except when 4,4'-oxydianiline (4,4'-ODA) was used as a diamine component. A good linear relationship was obtained between the amount of cyclodehydration reagent added and the extent of imidization (%) in the partially imidized PAAs (pi-PAAs) obtained. The pi-PAA cast films (10 μm thick) displayed a considerably high light transmittance at 365 nm (> 85%) in addition to its adequately controlled alkali-solubility. The photosensitive films consisting of the pi-PAAs and diazonaphthoquinone (DNQ) as a photosensitizer provided good positive-tone patterns through uv-irradiation and subsequent development using a 2.38 wt% tetramethylammonium hydroxide (TMAH) aqueous solution. The fully imidized films showed very high glass transition temperatures exceeding 300oC in some cases, sufficient film-forming ability (flexibility), and excellent transparency. The use of a thermally crosslinkable mono-functional end-capper, nadic anhydride (NA), was very effective to inhibit gelation during the chemical imidization process even in the H"-PMDA/4,4'-ODA system owing to its adequately controlled molecular weight. In addition, thermal curing of the NA-terminated system significantly improved the film toughness.