Evaluation of via density and low-k Young’s modulus influence on mechanical performance of advanced node multi-level Back-End-Of-Line
Improvement in electrical performance of the BEOL due to the introduction of ultra low-k dielectrics which comes at the cost of a reduced mechanical strength, weakening the whole BEOL. This is critical, as the BEOL is subjected to high thermo-mechanical stresses induced by the different coefficients of thermal expansion (CTE) between the silicon die containing Cu/low-k interconnects and the package during different processing steps, leading to structural failures in the BEOL. Tasks: Utilizing Design Of Experiments (DOE) and a decision making procedure, the mechanical performance of different advanced node Back-End-Of-Line (BEOL) configurations is evaluated, where the average peel stress in metal vias is considered as the critical parameter, and is identified with mechanical performance. The goal is to guide the design of the BEOL in its conceptual phase, with respect to the via densities and low-k Young’s modulus. Action: The procedure consists of three steps; first, a problem definition, secondly, an analysis of FEM obtained peel stresses using orthogonal polynomials for the regression analysis (as well as developing stress response surfaces), and finally the mechanical evaluation of different BEOL configurations by comparing them to the ideal BEOL configuration. Results: Regression analysis showed that low-k Young’s modulus has the biggest influence on stresses in individual via layers, between 1000 MPa and 5000 MPa, followed by the via density in those layers. Additionally, a decision making procedure has been outlined and applied to help to guide the choice of selection of the best BEOL configuration.