Stress-induced voiding in stacked tungsten via structure

The stress-induced voiding (SV) in Al-alloy films with stacked tungsten via structures was investigated with using new test structures. Voids were found in interconnections with stacked and borderless vias that had resistance increase after aging tests. Failure occurs most frequently when the test structures are stored at around 250 degrees C. This behavior can be explained by the diffusion creep model just like SV in a flat line [1]. A model of SV was obtained with thermal stress simulation and transmission electron microscopy (TEM) observation. Stress increases between upper and lower plugs with temperature increase over 175 degrees C.. Grains, which have high-angle misorientation, are often found above plugs. The tensile stress and grain misorientation should accelerate the void growth. O-2 plasma post metal etch treatment was found to be effective to eliminate SV in stacked via structure.