Mechanisms for Microstructure Evolution in Electroplated Copper Thin Films Near Room Temperature

We present a model which accounts for the dramatic evolution in the microstructure of electroplated copper thin films near room temperature. Microstructure evolution occurs during a transient period of hours following deposition, and includes an increase in grain size, changes in preferred crystallographic texture, and decreases in resistivity, hardness and compressive stress. The model is based on grain boundary energy in the fine-grained as-deposisted films providing the underlying energy density which drives abnormal grain growth. As the grain size increases from the as-deposited value of 0.05-0.1 µm up to several µm, the model predicts a decreasing grain boundary contribution to electron scattering which allows the resistivity to decrease by tens of percent to near-bulk values, as is observed. Concurrently, as the volume of dilute grain boundary regions decreases, the stress is shown to change in the tensile direction by tens of MPa, consistent with the measured values. The small as-deposited grain size is shown to be consistent with grain boundary pinning by a fine dispersion of particles or other pinning sites. In addition, room temperature diffusion of the pinning species along copper grain boundaries is shown to be adequate to allow the nucleation of abnormal grain growth after an initial incubation time, with a transient time inversely proportional to film thickness.