Simulations of damage and fracture in ULK under pad structures during Cu wirebond process

Abstract

Mechanical integrity of the dielectric stack is challenged by the trend towards porous, lower dielectric constant interlayer dielectric (ILD) materials. As a result, fracture in the ILD stacks caused either by assembly process or by the dicing process is an important reliability consideration. In general, there is a need to either assess the propensity of the structure to fracture under assembly conditions, or to design crack-arrest features that prevent propagation of cracks into active areas. In the case of wire bonded packages, the reliability concern associated with the fracture of Ultra Low-k (ULK) dielectrics while bonding over the active circuits (BOAC) is a significant challenge due to the impact load and the high ultrasonic energy transmitted to the ILD stack. In this paper, a multi-level modeling procedure is presented to assess the risk of fracture in ILD stacks during wire bonding process. First, a nonlinear, dynamic finite element model is developed to simulate the process steps - impact stage and last cycle of ultrasonic vibration and study the mechanical response of the ball, pad, and the underlying ULK under pad during copper wire bonding. Further, a simulation framework based on enriched isogeometric approximations is presented to compute damage accumulation in the ULK stacks using a cohesive damage description. The simulation framework is employed to develop insights on the potential crack initiation sites within the ILD stack and to evaluate the risk of fracture during each process step.