Polymeric coatings on copper surfaces are known to degrade at a faster rate than identical materials on other metals such as aluminum, particularly during thermal aging. We have studied the interfacial reactions occurring at copper surfaces coated with poly(esterimide) and polyimide wire enamels. Thin coatings (100 Å–1 μm) were heat treated at temperatures from 200 to 240 °C. Interfacial reactions were studied by x-ray photoelectron spectroscopy (XPS), Auger profiling, and reflectance infrared spectroscopy. In addition to copper oxide growth at the interface, thermal oxidative degradation of the polymer leads to thinning of the coating. This reaction is catalyzed by the copper (oxide) surface and material loss occurs primarily at the polymer/copper (oxide) interface. Migration of mobile copper species into the bulk of the coating is observed by its appearance at the surface and by depth profiling. For polyimide films prepared from poly(amide-acid) precursors, an interfacial reaction occurs during initial contact, prior to curing or aging, and infrared and XPS measurements show incomplete curing of the film. The formation of a copper carboxylate is postulated.
Thin films of polyimide were prepared by spin coating the poly(amide–acid) precursor onto copper and aluminum substrates, followed by the usual heat treatment to promote imidization (curing) of the film. Films prepared on aluminum substrates were completely cured during the heat treatment, as shown by x-ray photoelectron spectroscopy and infrared measurements. On copper substrates, the thinnest films (2000 Å or less) showed considerable intermixing of copper ions in the polymer layer. This prevented the films from curing completely during heat treatment. The formation of a copper carboxylate at the acid site in the polyimide precursor is postulated.
The degradation of poly(ester imide) films on metal substrates was monitored by determining the changes in polymer film thickness using x-ray photoelectron spectroscopy combined with ion sputter depth profiling. Thin films (1000–7000 Å) of poly(ester imide) wire enamel were spin coated onto metal substrates and aged in air at 200 °C for several hours. For films on copper substrates, thermal oxidative degradation of the polymer led to thinning of the film (2.5–3 Å/min), whereas films on aluminum substrates did not exhibit a decrease in thickness. The catalytic effect of copper upon the degradation of the polymer occurs primarily at the copper (oxide)/polymer interface, as revealed by the location of an inert gold marker layer within the polymer film before and after aging.
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