An Assessment of Risk of Fracture During Wirebond Over Active Circuits on ULK Dies

Abstract

Mechanical integrity of the dielectric stack is challenged by the trend toward porous, lower dielectric constant interlayer dielectric (ILD) materials. During copper wirebonding over active circuits, the fracture of ultralow-k (ULK) dielectrics is a significant reliability challenge due to both the impact load and the high ultrasonic energy transmitted to the ILD stack. Thus, there is a need to assess the propensity of the ILD to fracture under the wirebonding process conditions. In this paper, a numerical investigation of the risk of fracture in ULK dies due to the Cu wirebonding process is presented. A recently developed computational technique for modeling cracks using behavioral approximations built directly on the geometrical model (enriched isogeometric analysis), as opposed to the analysis of a meshed geometry, is adopted to model damage in the ULK stacks. The cohesive damage induced in the ILD stack during the process steps, which is a measure of the risk of crack initiation, is used as an indicator of the reliability risk. The damage accumulation during the Cu wirebonding process is estimated to identify weak interfaces and potential sites for crack nucleation as well as damage nucleation patterns. Furthermore, the critical process condition at which crack nucleation is likely is identified by analyzing the damage induced during the impact and ultrasonic excitation stages. Also, representative ILD stack designs with varying Cu percentage are compared to assess the relative risk of fracture.