Numerical Model for Understanding Failure Mechanism of Back End of Line (BEOL) in Bump Shear

Abstract

With the increasing requirement for advanced technology nodes in high-performance devices, low-K (LK), ultralow-K (ULK) and extreme low K (ELK) dielectric materials have been integrated with copper pillar bumps in Back-End-of-Line (BEOL) interconnects in flip chips to reduce capacitance and thus resistance-capacitance (RC) delay. The dielectric materials (LK, ULK and ELK) are fragile compared to silicon oxide. Due to large coefficient of thermal expansion (CTE) mismatch between the die and substrate, dielectric crack and delamination can be induced by the stress transferred by the stiffer copper pillar bumps during chip attach reflow. It is thus important to characterize silicon BEOL strength for chip-package-interaction (CPI) qualification.Bump shear provides a quick method to characterize the silicon BEOL strength by shearing the copper pillar bumps. However, the failure mode in bump shear test is complex and poses a challenge for understanding the failure mechanism. It was observed experimentally that the failure mode induced by bump shear depends on the shear height. Generally, brittle dielectric failure can be induced with a higher shear height. In contrast, ductile Aluminum pad (AP) failure can occur with a lower shear height. A mixed failure mode of both dielectric failure and AP failure may also occur, making the failure mechanism more complex. Furthermore, it was found that metal layout in the BEOL could also affect the critical shear force in the shear load curve.This paper presents a numerical model for better understanding the failure mechanism of BEOL in bump shear. First, a stress model is developed to understand the stress distribution in bump shear. In this work, a contact/target element model is used to simulate the interaction of shear tool surface with the bump surface and a cohesive zone material (CZM) model is used to simulate the potential interface debonding. Plasticity of copper pillar is also considered to capture the plastic energy dissipation. Therefore, the bump/shear tool interaction, energy release due to crack propagation and copper plastic energy dissipation can be well captured in the model. Then an ELK fracture model is also developed to understand the BEOL crack failure. The impact of metal density on ELK crack is evaluated by comparing the energy release rate at the crack tip. The impact of dielectric material properties on the ELK crack is also evaluated. The numerical models provide a good understanding of the stress distribution and BEOL fracture mechanism in bump shear.