In-situ studies of electromigration voiding in passivated copper interconnects

Extensive research has focused on understanding the complex electromigration behavior of aluminum-based interconnects. With the trend toward smaller device geometries continuing, copper is becoming more widely used due to its lower resistivity and improved electromigration resistance. This necessitates a better understanding of electromigration void dynamics in copper interconnects. In-situ studies of electromigration in a high-voltage SEM (120 keV) enable observation of void behavior through the passivation layer. Experiments show that electromigration voids in aluminum-based interconnects nucleate at the sidewalls of the lines and appear to extend through the thickness of the lines. The voids move along the sidewalls and change shape, eventually growing across the line to cause failure. The dominant diffusion mechanisms are likely to be grain boundary diffusion for nucleation and growth and surface diffusion along the internal surface of the void for void motion. Electromigration voids in copper lines, however, behave very differently. Voids often span the width of the line early in an electromigration test, but do not extend through the thickness of the line, appearing instead as thinned regions on the surface. Interface diffusion seems to be the dominant mechanism as voids move along the line. Failure occurs only after a void grows through the thickness of the line. This study will examine the electromigration behavior of copper damascene interconnects, comparing the void behavior to that in aluminum interconnects. Qualitative observations will focus on the void shape and morphology.