In‐plane solvent diffusion in a photosensitive polyimide

The diffusion of an organic solvent, N ‐methylpyrollidinone (NMP), in a fully imidized photosensitive polyimide, 4,4′‐benzophenone tetracarboxylic dianhydride (BTDA– t MDA), was investigated. The polyimide (PI) films were prepared by spin‐coating, soft‐bake, deep UV exposure, and final‐baked to a peak temperature ranging from 230 to 375°C. The thickness of the final films is approximately 5.0 μm. Patterned metal was then electronbeam‐evaporated through a molybdenum shadow mask over the PI surface. The rate of lateral (in‐plane) diffusion of NMP in the PI covered with metal follows Fick's \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt t $\end{document} relationship over a temperature range of 40–85°C. The rate of diffusion was shown to increase with increasing NMP temperatures, typical of a thermal‐activated process. The diffusion coefficients, as measured by the solvent penetration distance as a function of the square root of time, were found to obey the Arrhenius equation: D = D 0 × exp(− Q / RT ) cm 2 /s, where the frequency factor, D 0 , ranges from 0.0015 to 0.0045 cm 2 /s, and the activation energy, Q , ranges from 5.52–6.34 kcal/mol. Both D 0 and Q decrease with increasing PI baking temperatures. © 1992 John Wiley & Sons, Inc.