Surface engineering via alloying opens up a new route to tune the strength of an alloy film, thus it is critical to determine an optimal alloy addition. Here, the effect of varying the content of extremely dilute manganese oxide (MnO), namely 0.04, 0.10 and 0.17%, on microstructure stability and reliability of electroless plated colloidal Cu alloy thin films upon annealing is studied. An intermediate MnO content (0.10%) noticeably gives the best performance in terms of stabilizing Cu grains and enhancing film reliability against thermal stressing. Transmission electron microscopy and secondary ion mass spectroscopy reveal that the strengthening mechanism is related to interfacial segregation of MnO (serving as a barrier) and grain-boundary segregation of MnO (pinning and stabilizing Cu grains). However, the stabilization of Cu grains by the segregation of MnO are both missing from the other two films with MnO additions of 0.04% and 0.17%, and an identical annealing thus leads to their premature failure. Thin-film properties of adhesion strength, residual stress, surface energy, and MnO segregation tendency are comprehensively studied elucidating the strengthening mechanism.
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