A novel positive photosensitive polybenzoxazole precursor for microelectronic applications

A positive working, aqueous base developable photosensitive polybenzoxazole (PBO) precursor composition based on a partially trimethylsilyl (TMS) protected PBO precursor and a bisphenol A based 1,2-naphthoquinone diazide-4-sulfonate (DNQ-4) photosensitive compound has been developed. The polymer was prepared from a low temperature polymerization of 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (BisAPAF) and isophthaloyl chloride (IC), followed by reacting with trimethylchlorosilane. Subsequently, thermal cyclization of the PBO precursor at 350 °C produced the corresponding thermally stable PBO. The inherent viscosity of the precursor polymer was 0.35 dl/g. The cyclized PBO showed a glass transition temperature (Tg) at 309 °C and a 5% weight loss at 550 °C in nitrogen. The structure of the precursor polymer and the fully cyclized polymer were characterized by FTIR and 1H NMR. The photosensitive PBO precursor containing 20 wt% DNQ-4 photosensitive compound showed a sensitivity of 172 mJ/cm2 and a contrast of 1.33 in a 3-μm film with a 0.6 wt% tetramethylammonium hydroxide (TMAH) developer. A pattern with a resolution of 5 μm was obtained from this composition. The novel PBO precursor photosensitive composition showed a significant improvement in dark film loss after development and could be used to make a thick film resist.