Polybenzoxazoles (PBO) are also expected as low-K & low-CTE dielectric materials as well as polyimides. High molecular weight PBO precursors, polyhydroxyamides (PHAs), were prepared from silylated bis(3-hydroxy4-amino)biphenyl(p-HAB) and dichlorides of terephthalic acid (TPA) or trans-1,4-cyclohexanedicarboxylic acid in DMAc/ITMPA in the presence of pyridine and LiCl. After precipitation of the PHA solution into water to remove the salts, it was difficult to re-dissolve into salt-free aprotic solvents. Therefore, the salt-containing PHA solution was cast on a glass plate and washed with water to remove salt, then the dried films fixed in a frame were heated at 400degreesC/1h to ensure complete transformation to PBO. The obtained PBO films showed a much lower K, a lower CTE and a higher Tg than the corresponding PIs. The use of a fluorine-containing bis(o-aminophenol) as a comonomer for the homo p-HAB/TPA system improved significantly the solubility into salt-free solvents. Another PHA derived from an isomer of p-HAB, bis(4-hydroxy-3-amino)biphenyl (m-HAB) and TPA also showed a high solubility. These new PBO systems made possible the convenient film formation process (cast-drying-thermal treatment like PI systems) for microelectronic applications. Simultaneously, these PBO films maintained comparatively low K values, considerable low CTEs, and high Tg's as in the homo p-HAB/TPA systems.
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