A copper reflow model implemented in a feature profile evolution model will be discussed. The barrier/seed/plating process flow is being challenged as the metal interconnects shrink in size. The barrier and seed deposition are requiring more of the total volume within the interconnect via and trench structures. Copper reflow is being explored as a means to fill via and trench structures for back end of line interconnects. The wafer is raised in temperature to allow copper to move, shift around, or "reflow" within the feature. The feature model used for the study is a 3D Monte Carlo model. The reflow model implemented here assumes Cu transport during reflow is due to surface diffusion. The surface diffusion depends on the wafer temperature, the material Cu is diffusing on (i.e., Cu, Ta, SiO2, etc.), and the local morphology of the surface. The implemented surface diffusion model is a hopping model. A probability matrix is constructed to determine the next surface "hop" location as the Cu moves around on the surface. The hopping Cu will settle into surfaces which minimize surface energy. In this model that translates to Cu preferring higher coordination number (CN) sites. Depending on the temperature and material parameters the minimal surface morphology can change. If hopping probabilities are similar between CNs the surface will roughen and become more dendritic. If there are large hopping probability differences the surfaces will become more faceted with low energy crystalline planes exposed. Using a simple hopping surface diffusion model, reflow behavior is shown. The model predicts the initial reflow causes rounding of the Cu surfaces and a shrinking of the opening as the surfaces round to a more minimal surface configuration. The end result for the successful reflow cases is a filled feature which is concave at the feature bottom or flat once the field is reached. The main issue with the reflow process is pinch-off at the feature opening causing void formation in the feature. For vias the model is predicting mostly void formation. One reason vias are more prone to void formation (vs trench) is the different CN distributions for a flat (trench) vs rounded (via) surface. Thus, the movement (i.e., hopping) of the Cu is different for these surfaces. Another reason is trenches are closing in on the opening from only two sides whereas the via is closing in from all sides. Once a void forms it is unlikely to breach the field and have more copper enter the feature. Smaller voids move more quickly about the feature vs large voids given the smaller amount of Cu needed to shift in the small void for the void center to change position.