Knowledge of the geometry effect on impurity incorporation and grain growth in narrow lines is important for reducing copper line resistivity. In this paper, we investigate impurity incorporation in narrow lines with time-of-flight secondary ion mass spectroscopy. We also study the influence of linewidth, trench depth, pattern density, and overburden on copper grain growth in reduced dimensions. The concentration of chlorine and carbon is found to increase with decreasing linewidth, while the concentration of sulfur is close to the detection limit. This effect contributes to copper superfilling and is consistent with the curvature-enhanced accelerator coverage model. Copper self-anneal slows down as linewidth decreases, although an opposite trend is observed for lines below a width of about 300 nm. Impurity incorporation and geometric constraint retard copper grain growth in narrow lines, resulting in an inverse relationship between copper line resistivity and geometry. However, copper overburden, which has a much larger grain size, can enhance grain growth in narrow lines depending on the line geometry. Reducing impurities and balancing trench depth and copper overburden can be used to reduce the resistivity of narrow copper lines.