On the Elastic Assumption for Copper Lines in Interconnect Stress Modeling

Finite element modeling of thermal stresses in copper (Cu) interconnect lines is carried out to examine the effect of input material model on the stress-temperature output. This paper is motivated by the near-linear response of experimentally measured stress in passivated Cu lines as a function of temperature, which often drew the conclusion that Cu lines behave only elastically. In the present analysis, a purely elastic Cu and an elastic-plastic Cu are employed in the interconnect structure containing the metallization, the barrier layers, and the dielectric. Thermal cycling between 20degC and 350degC is simulated. It is shown that both material models for Cu yield similar stress-temperature histories. However, with consideration of available experimental observations and further numerical analyses, it is shown that the elastic-plastic nature of Cu lines needs to be taken into account in interconnect stress modeling. Select simulation results on Cu interconnect/low-k dielectric systems are also presented.