Numerical Analysis of the Reliability of Cu/low-k Bond Pad Interconnections Under Wire Pull Test: Application of a 3D Energy Based Failure Criterion

Due to size reduction in die manufacturing and introduction of brittle dielectric materials, crack related failures occur currently, mainly in interconnect levels. By means of Finite Element (FE) simulations, an energy based failure criterion named Nodal Release Energy (NRE) Method, inspired by the so-called Area Release Energy (ARE) one developed by Philips Applied Technologies, is used to numerically predict the mechanical related failures. More precisely, the failure index is applied to investigate wire bonding induced peeling. In this paper, the NRE method is presented and its added value to forecast delamination failures in a typical microelectronics stack is demonstrated. The NRE method is related to fracture mechanics and founded on propagation approach. Two FE calculations are used to evaluate the energy quantity: an uncracked and cracked one. In the latter model, a virtual crack is inserted. Aiming to compare the NRE values with known physical quantities experimentally measured such as critical adhesion energy, a relation bridging the gap from NRE to the Energy Release Rate is given. This relation is based on the crack extension method and relates to Griffith theory. The accuracy of the NRE method is investigated through comparisons with 2D and 3D analytical cases. Results show that the method provides a good approximation. The NRE behaviour with respect to key numerical parameters will be studied. At last, a typical bond pad structure under a wire pull test is simulated. Both stress and energy based analyses are carried out. The critical interface is investigated with both post processing methods. Results based on the energy criterion show that delamination interface is in agreement with experimental observations, in contrast to stress based values. However, it is also shown that simulation results can depend on the prescribed crack length, suggesting a accurate definition of the cracked model. The main assumptions done in this study are discussed, trying - - to define the associated uncertainties, particularly residual stress and crack morphology features. Finally, the added insights provided by NRE method and its ability to help in design and process development for advanced IC technologies are demonstrated.