To improve the speed of integrated circuits it is highly important to minimize the electrical resistivity of their interconnects. However, as the dimensions of the interconnects approach the mean free path of the electrons, a substantial rise in resistivity occurs due to additional electron scatterings from grain boundaries and interfaces. To investigate the role of interfaces, in-situ resistivity measurements were preformed for thin copper films on which different materials were deposited. The resistivity was monitored before and after the deposition, enabling to observe the changes as a new top interface was created. Among the materials tested, tantalum resulted in the highest resistivity increase. This fact is of special importance since this material and its nitride serve today as the common diffusion barrier for copper metallization. Titanium resulted in a smaller resistivity increase, but still higher than that of a free copper surface in vacuum. The developed approach enables to test the influence of different diffusion barriers on copper resistivity. With the continuously shrinking dimensions of copper interconnects, this factor will have an increasing importance in future technology nodes.