The evolution from Cu to Ta surface during Cu CMP has been investigated. As a first step of Cu CMP, removing excess Cu on field area to Ta often gives rise to topography issues such as dishing or erosion. The exposure of Ta is detected by in-situ reflectance measurement, where the downward slope-start signifies the beginning of Ta exposure. For blanket wafers, the maximum remaining Cu thickness at slope-start increases with initial Cu thickness. Applying lower polishing pressure and table speed, so-called soft landing, near the Cu-Ta transition stage results in thicker remaining Cu. In patterned wafers, Ta exposure starts near high pattern density areas such as wide lines, and contrary to blanket wafers, remaining Cu thickness at slope-start cannot be correlated simply with initial Cu thickness. In this experiment, with increasing initial Cu thickness, remaining Cu thickness at slope-start decreased. The amount of dishing is largely proportional to the initial Cu thickness.