The effect of material properties and initial defects on the thermo-mechanical behavior of a dual damascene module

For backend processes, thermo-mechanical failure is one of the major failure modes. A representative metal structure in a Cu/low-k dual damascene process is examined, considering the major thermal loads and process steps through combined FEA with experiments. Firstly the low-k material, in our case the polymeric material SiLK (trade name of the DOW Chemical Company), is characterized and modeled to provide a reliable material model and data for the simulations. The coefficients for a Maxwell relaxation model are calculated, temperature dependency and its influence on the modulus are examined and the WLF coefficients are calculated, providing time and temperature dependent material parameters for the process simulations. The main dual damascene process steps are simulated using the obtained material model. Stresses are examined at different critical locations. Furthermore, an initial defect is placed at a low-k-oxide interface, where energy release rates are determined. Our results show that Cu/low-k structures exhibit significantly different reliability characteristics than their Al predecessors, and are more critical from several design aspects. This not only makes the stress management in the stacks more difficult, but also strongly impacts packaging.