By James A. Cunningham, Consultant, Saratoga, California

Early failures at the vias are severely limiting current density rules in today’s copper dual-damascene interconnect system. Recent work shows that if the barrier at the base of the vias were removed or made substantially thinner, or if interface diffusion rates were brought under control, current density rules would be escalated to much higher values.

Some twenty years ago, when the industry first began to seriously consider copper as a replacement for aluminum alloys, it was assumed that circuit designers would finally get what they had been asking for-much higher current density design-rules for interconnects. Considering the massive development task ahead, process engineers thought that providing such an improvement would be the easy part. After all, at 125 degrees C, for example, the diffusion rate of copper within itself is a billion times slower than that of aluminum.

The betting was that the old rules, where current densities had to stay below about 10⁵A/cm², would soon be swept away by the new metal. But these early prognostications were based on comparing lattice diffusion rates. Unfortunately, this is not what paces the rate of copper transport at high current density in today’s copper system. The metal is not moving within each little grain, or even at the grain boundaries, as aluminum primarily does, but at the interfaces between the copper and the diffusion barrier metal, and between the copper and the silicon nitride cap. These boundaries have no real metallurgical or chemical binding, and subsequently, adhesion is limited. Like the shifting grains of sand on the surface of a desert, the metal seems free to move, and current density design-rules for copper, at approximately 10⁶A/cm² or less, are far below its promise.