Deformation of interconnect structures at the back-end of microelectronic devices during processing or service can have a pronounced effect on component reliability. Here, we use atomic force microscopy (AFM) to study plastic deformation and interfacial sliding of Cu interconnects lines on Si. The behavior of both stand-alone Cu lines and lines embedded in a low K dielectric was studied. Following thermal cycling, changes were observed in the in-plane Cu line dimensions, as well as the out-of plane step height between Cu and dielectric in single layer structures. These were attributed to differential deformation of the Cu/Si and Cu/dielectric material pairs due to thermal expansion mismatch, accommodated by interfacial creep. These results are discussed in light of previous work on the mechanism of interfacial creep. Some preliminary results on the distortion of Cu lines due to package-level stresses are also presented.