Polyimides (PI) are an important class of high temperature engineering plastics. Their combination of thermal stability, low dielectric constants, and low coefficients of thermal expansion make these materials desirable for many applications in the semiconductor industry. One of the challenges with PI is the lack of solubility and the consequent need to process the materials as precursor solutions, followed by thermal curing at 300-400°C. This high temperature cure step complicates PI integration into many chip packaging applications and currently requires use of inferior, lower temperature thermosets.<br/>While it is known that organic bases can lower the curing temperature of PI, adding them to PI solutions quickly causes gelation and ruins the shelf life. We have discovered that many organic and inorganic salts will also affect imidization temperature. The meta ethyl ester polymer of pyromellitic dianhydride and 2,2’-bis(trifluoromethyl) benzidine was chosen as the testbed due to its improved solution stability and material properties compared to traditional polyimide precursors. Imidization was monitored by IR spectroscopy and it was found that no imidization of the polymer occurs below 200°C in the absence of catalyst. However, addition of neutral inorganic salts such as LiCl were found to cause significant imidization, but with an onset temperature above 160°C.<br/>The catalytic activity of phase transfer catalysts, ionic liquids, and alkylated amidine salts were also explored. 1-butyl 3-methylimidazolium (bmim) and alkylated DBU salts were found to be inert below 150°C, but completely imidized films within minutes at 220°C. DSC and TGA data show that the salts are not decomposed at these temperatures, so the salt itself acts as the catalyst. Finally, nano-indentation shows that the modulus is higher in low temperature cured samples than in traditional thermally cured films.

Low temperature imidization of poly(amic ester) by organic and inorganic salts

251st National Meeting (2016)

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