Void formation mechanism in VLSI aluminum metallization

Void formation is aluminum lines caused by high-temperature heat treatment has been investigated, particularly from the aspect of mechanical interaction between the passivation layer and aluminum lines. It is found that there are two effects causing voids in aluminum lines: the deformation of the passivation layer and the so-called thermal expansion mismatch between aluminum lines and their surrounding layers. While the thermal expansion effect is independent of aluminum line dimensions, the passivation deformation effect dominates preferentially in wide aluminum lines. Bulge deformation of the passivation layer is caused by its own compressive stress, forcing the encapsulated aluminum to expand. In fine lines, void formation is controlled by the volume difference caused by the thermal expansion mismatch and aluminum diffusion