Influence of visco-elasticity of low-k dielectrics on thermo-mechanical behavior of dual damascene process

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 finite element simulation 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. Characterization measurements (nano-indentation-creep test) are carried out on a polymer film deposited on a substrate. Here a quasi-elastic approach is used to account for the substrate influence and the time dependency acting at the same time. Elastic indentation curves are simulated with a varying modulus of the film within an expected interval. The coefficients for a Maxwell relaxation model are calculated, and verified through FEM simulations. Furthermore results of temperature dependency and 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 aluminum predecessors, which are more critical from several design aspects. This not only makes the stress management in the stacks more difficult, but also strongly impacts packaging.