The effectivity of cryo-scanning transmission electron microscopy-electron energy loss spectroscopy was demonstrated for nanoscale analysis of the cross-section of the Cu/polyimide interface. The nanoscale Cu/Cu2O/CuO layer structure at the interface was clearly observed for the first time. In addition, a Cu atom was identified, embedded in the polyimide matrix, and the average valence of diffusing Cu atoms or nanoclusters was determined using (cryo-)scanning transmission electron microscopy-electron energy loss spectroscopy. On the basis of these results, we have proposed a mechanism for the diffusion of Cu atoms in polyimide. To the best of our knowledge, this is the first report of the observation of a metal atom embedded in an insulating amorphous polymer.

With commercialization of the fifth-generation mobile communication system and the further spread of the Internet of Things, industrial innovation is arriving with new business fields related to concepts such as high-speed communication, self-driving vehicles, and remote medicine. One of the challenges is the realization of flexible devices with high-definition circuits, which requires new fabrication techniques for Cu films on polymer substrates to meet demands and an understanding of Cu/polymer interfacial nanostructure to assure product quality. We have developed a promising technique for the fabrication of Cu film on polyimide (PI), which consists mainly of very simple semiconductor device processes. This technique allows for control of the Cu thickness with nanometer precision to form miniaturized Cu circuits with potential advantages in terms of interfacial adhesion and material/production costs. The Cu/PI interfaces fabricated by conventional vapor deposition and the new technique are systematically analyzed using synchrotron hard X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy. With conventional vapor deposition, it was discovered that evaporated Cu atoms decompose the PI and an oxidation layer with a thickness of several nanometers that deteriorates the interfacial adhesion could be visualized at the Cu/PI interface. With the new technique, the decomposition of PI and interfacial oxidation are significantly suppressed. Furthermore, the proposed technique can be broadly applied to the investigation of metal/polymer interfaces fabricated by polymer coating on a metal substrate, which has so far been impossible.

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