A novel positive‐type photosensitive polyimide based on soluble block copolyimide showing low dielectric constant with a low‐temperature curing process

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Abstract
A soluble block copolyimide (Bco‐PI) was prepared by direct one‐pot polycondensation of 2,2‐bis‐(3‐amino‐4‐hydroxyphenyl)hexafluoropropane, 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and cyclohexane‐tetracarboxylic dianhydride in the presence of γ‐valerolactone and a pyridine catalyst system using γ‐butyrolactone as the solvent. The thermal transmission temperature (Tg) of Bco‐PI was 282°C. By having the curing process occur at 250°C, which was below the Tg, colorless and transparent films could be obtained. The film showed excellent optical characteristics. Such properties could not be attained by the conventional high‐temperature thermal imidization process of poly(amic acid). The hydroxy groups in the polyimide backbone gave the Bco‐PI the potential to become alkaline. To the Bco‐PI was added 15 wt % ester of 2,3,4‐trihydroxybenzophenone with 1,2‐naphthoquinone‐(2) diazide‐5‐sulfonic acid (NT200) as the photoreactive compound. The system worked as a positive‐type photosensitive polyimide (PSPI). The sensitivity and contrast of the PSPI system were 220 mJ/cm2 and 1.27, respectively, when exposed to UV light, followed by development with a 5% tetramethylammonium hydroxide (TMAH) aqueous solution at room temperature. After curing at 250°C for 1 h, the average refractive index of Bco‐PI with and without NT200 was 1.5543 and 1.5563, and the optically estimated dielectric constant of the polyimides was 2.66 and 2.67, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4240–4246, 2006

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