Crack initiation in Cu-interconnect structures

Resistance to the brittle cracks of single damascene line patterns on silicon wafers is investigated through three points bending tests. After loading, samples are observed by atomic force microscopy or field emission gun scanning electron microscopy, in order to detect cracks. For given load and pattern period, an appropriate numerical procedure allows calculation of the characteristic parameters of all the stress singularities, which appear along the dihedral lines at interfaces between the various materials. The most critical singularity is determined in terms of crack initiation according to a criterion combining stress intensity and energy release rate conditions. In these interconnects, crack initiation appears to be intrinsically governed by energy release conditions, so that short cracks are very difficult to initiate or propagate. The absence of detectable cracks after bending is in good agreement with this prediction.