A new regime of hillock growth has been observed in patterned Al98.5 W.%Si1.0 Wt.%-Cuo0.5 wt.% films. The “surface” hillock and “side” hillock, which have been seen previously, form on patterned metal lines having linewidths greater than the larger Al alloy grain sizes (∼3 μm). None is seen on the fabricated lines having linewidths between 0.9 and 2 μm where long-range grain boundary diffusion cannot occur because of its bamboo structure. However, a new type of hillock, the “line hillock”, occurs in structures having linewidths of 0.6 μm. The presence of this last type of hillock is inconsistent with the current understanding of hillock formation and may present severe restrictions on the down-sizing of ultra–large–scale integrated devices.