Stress modeling of Cu/low-k BEoL - application to stress migration

Stress migration (SM) or stress-induced voiding (SIV) experiments were conducted for two BEoL (Back End of Line) technologies: Cu/FTEOS and Cu/Low-k. Experiments have shown the mean time to failure (MTF) depends on ILD (interlayer dielectric) 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 (inter-layer dielectric) properties, ILD stack and metal line width. The concept of a SM Risk Index is proposed to assess BEoL stress migration reliability from the stress perspective. 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.