Delamination prediction in stacked back-end structure underneath bond pads

Abstract

The thermo-mechanical reliability of integrated circuits (ICs) gains importance due to the reducing feature sizes and the application of new materials. This paper focuses on the delamination in the stacked back-end structure underneath bond pads. Current simulation tools predict this failure mode following a linear elastic fracture mechanics approach; whereas an interface damage mechanics (IDM) approach would be more appropriate to our opinion. The basics of IDM by cohesive zone modeling are outlined. The cohesive zone finite element under consideration is a two-dimensional (2D) linear element for small deformations with an exponential traction separation law. A 2D plane strain model represents a simplified microstructure underneath a bond pad. Several finite element (FE) meshes are constructed with gradually decreasing mesh sizes along the interfaces. Furthermore, two cohesive zone parameter sets are considered, one for 'weak' adhesion between the material layers and one for 'strong' adhesion. The simulations with the FE models demonstrate the capability of IDM to simulate the damage evolution, where several interfacial cracks develop simultaneously. The effect of mesh refinement is illustrated. It improves the convergence of the applied nonlinear solution procedure. Furthermore, the correlation between the adhesion strength and the complexity of the equilibrium path is shown. Finally the conclusions are drawn for the current research and recommendations are given for the further development of IDM applied to delamination prediction in IC back-end structures