Simulation and experiments of stress migration for Cu/low-k BEoL

Stress migration (SM) or stress-induced voiding experiments were conducted for two back-end-of-line (BEoL) technologies: Cu/FTEOS and Cu/low-k. Experiments have shown the mean time to failure (MTF) depends on inter-layer dielectric (ILD) materials properties, ILD stack and metal line width. Stress migration is worse in Cu/low-k, manifesting as significantly reduced MTF under accelerated testing. Line width also has a more profound effect on stress migration reliability in Cu/low-k than in Cu/FTEOS. Wider lines produce higher failure rates, due to larger stress magnitudes in Cu and larger active diffusion volumes. Stress modeling using finite element analysis (FEA) was performed to quantify the stress fields in the via-chain test structure used for SM reliability testing. In order to account for the effect of process steps on stress evolution, a process-oriented modeling approach was developed. Stress in the metal line is a function of ILD properties, ILD stack and metal line width. The concept of an SM risk index is proposed to assess BEoL stress migration reliability from both stress and energy perspectives. Comparison of the SM risk index for Cu/FTEOS and Cu/low-k shows that the latter is more prone to stress-induced voiding. Stress migration tests verify that MTF values decrease with increasing line width. Modeling results are consistent with experimental findings, while providing more insightful understanding of stress-driven mechanisms in stress migration.