Reliability Evaluation for Copper/Low-$k$ Structures Based on Experimental and Numerical Methods

Bump shear is widely used to characterize the interfacial strength of Cu/low- $k$ structures. In this paper, the blanket low-$k$ structure was used to evaluate the reliability and strength of Cu/low- $k$ structures based on experiment and finite-element modeling technique. The objectives of this paper are to determine the critical stress parameters for low-$k$ interfaces with different low-$k$ structures, to understand the failure mechanism, and to improve low-$k$ structure reliability by optimizing some parameters. In this paper, a comprehensive parametric study was carried out. Such parameters include the effect of three different low-$k$ structures, high-Pb solder bump versus Pb-free solder bump, different underbump metallization (UBM) thicknesses, barrier-layer material elastic modulus, and shear ram height on low-$k$ structure reliability. The simulation findings can be summarized as follows. The critical stress decreases with the number of layers of low-$k$ structure. An Sn–Ag solder bump results in a higher shear force and stress than a high-Pb solder bump. Reducing the UBM thickness can help improve the low- $k$ structure reliability.