In situ ultra-high vacuum transmission electron microscopy studies of the transient oxidation stage of Cu and Cu alloy thin films.

Because environmental stability is an essential property of most engineered materials, many theories exist to explain oxidation mechanisms. Yet, nearly all classical oxidation theories assume a uniform growing film, where structural changes were not considered because of the previous lack of experimental procedure to visualize this non-uniform growth in conditions that allowed for highly controlled surfaces and impurities. With the advent of vacuum technologies and advances in microcopy techniques, especially in situ, one can now see structural changes under controlled surface conditions. Here, we present a review of our systematic studies on the transient oxidation stages of a model metal system, Cu, and its alloys, Cu-Au and Cu-Ni, by in situ ultra-high vacuum transmission electron microscopy (UHV-TEM). The dependence of the oxidation behavior on the crystal orientation, oxygen pressure, temperature and alloying is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy and the interfacial strain energy. Heteroepitaxial concepts, developed to explain thin film formation on a dissimilar substrate material (e.g., Ge on Si), described well these initial oxidation stages.