The early stages of preexisting small void growth into larger voids of characteristic shapes caused by stress-induced migration in aluminum lines were studied through biaxial thermal stress simulation. Preexisting small voids, referred to as notches, were assumed to be hemicylindrical, triangular, and grain boundary edge notches. The author showed that the notches have the potential to grow into voids of varying shape such as wedgelike, grainlike, and slitlike voids. The void shape depends strongly on the stress concentration and the free surface boundary conditions. The stress concentration suppresses void growth. The free surface influences not only the shape of the voids but also their size. (c) 2007 American Vacuum Society.