Resistance Decay of Cu/Porous Low- ${k}$ Interconnect: Modeling and its Impact on Electromigration

It was found previously the resistance of Cu interconnect (R) could decrease during baking and acceleration test, owing to moisture invasion through the nanoporous low-k followed by moisture-induced oxidation of Ta-based liner and specularity recovery of the electron scattering at Cu/Ta interface. In this paper, a model is established to quantitatively interpret the underlying kinetics. It is shown that the model fits well with the experimentally observed hump-shaped temperature dependence of the R decay rate of Cu/porous SiCOH interconnects at 28-nm node at 170 °C~350 °C. It reveals that the liner oxidation and resistance decay are controlled mainly by the migration of metal cation through the as-oxidized liner at the low temperatures but by the desorption/diffusion of the chemisorbed moisture in the low-k at the high temperatures. Particularly, the model well reproduces the modest and unexpectedly significant low-k-thickness dependence of the decay rate at the low and high temperatures, respectively,which indicates clearly the temperature-dependent influence of the tortuous diffusion path of moisture in the porous low-k. The intimate relationship of Cu electromigration performance to such R decay is also demonstrated.