Abstract As the scale of Cu interconnects reduces to sub-100nm, the drastic resistivity increases result from the decrease of Cu thickness, which is called “size effect”. This thesis focuses on the two major factors of “size effect”- surface scattering and grain boundary scattering. The first part of the research attempts to corelate the theory of surface scattering with the theory of grain boundary scattering. By combining parameters of the two scattering effects, we can probe into the relation between the change of film structures and physical meaning of internal electrons more reliable. The second part of the research compares resistivity and microstructure of Cu thin films with barrier layers, which is made by different materials or different processes. Finally, resistivity and grain size of the samples are measured at low temperature as well as after annealing, in hopes of reducing the Cu resistivity increase and improving the integrated-circuit efficiency.