Thermo-mechanical Challenges of Cu/Low k Interconnects

As the interconnect scaling down to deep submicron regime, it begins to dominate overall RC delay of the device. This makes the transition of dielectric materials from SiO₂ to low-k essential. The drastic deterioration in mechanical properties of low-k materials has led to serious process challenges in both integration and assembly. As a result, the thermo-mechanical stability has emerged as a major reliability concern with interfacial delamination and ILD cohesive fracture being the leading failure modes. Another fundamental change of low-k ILD films is their tensile intrinsic stresses in comparison of compressive ones enjoyed by conventional SiO₂ based materials. This paper will review mechanical properties of low-k materials, describe a set of thin film characterization metrologies and review their merits based on low-k and ULK data. In addition, potential failure modes relevant to current interconnect feature size will be discussed along with some scaling trends moving forward.