X-ray photoemission investigations of clustering and electron emission, injection, and trapping at the gold/polyimide interface

Abstract

The evolution of the interface formed between clean [N-N-(p,p-oxydiphenylene) pyromellitimide] polyimide (PI) and evaporated gold was followed by high-resolution x-ray photoelectron spectroscopy (XPS). Overlayer and substrate core level emission behavior indicates the nucleation and growth of Au clusters on the PI surface. At low gold coverages (<1 Å nominal thickness), there was no observed interaction between the adatoms and the substrate. Above 1 Å we observe the growth of clusters on the PI surface and a shift of the Au 4f core levels as the clusters become metallic. Significantly, we observe further shifting of the Au 4f emission and the Fermi level at coverages greater than that expected for metallic behavior, and we see Au-cluster-induced broadening and shifting of the substrate core level spectra to lower binding energy. This unexpected behavior can be accounted for in terms of the injection and trapping of charge into the PI film from Au during the photoionization process. Angle-dependent XPS results showed increased broadening and shifting near the surface, indicating that the amount of trapped charge was greater at the interface. This injection profile was modified by an annealing cycle of the Au/PI interface, as indicated by the coalescing of dispersed Au clusters and the partial recovery of the clean PI substrate characteristics.