In Chapter 5, we show that the microstructure and interfaces are important in controlling EM reliability of Cu damascene interconnects where the EM lifetime can be significantly improved with metal capping or alloying. In this chapter, we investigate the scaling effect on microstructure and the implication on EM reliability for Cu and Co damascene lines. The scaling effect on Cu microstructure was investigated using a high-resolution electron microdiffraction technique down to 22 nm linewidth for the 14 nm node. The results showed a systematic trend of microstructure evolution in Cu damascene lines with continued scaling. A Monte Carlo simulation was carried out to investigate grain growth in Cu interconnects beyond 22 nm linewidth based on total energy minimization. The simulation results enabled us to understand how the interface energy counteracts the strain and grain boundary energies to control the microstructure evolution in Cu lines with continued scaling. Then the scaling effect on microstructure evolution of Co damascene lines was investigated beyond the 10nm node using both electron microdiffraction and simulation. The simulated microstructures of Cu and Co interconnects are used to project the scaling effect on EM reliability beyond the 10 nm node.