DIFFUSIONAL BACK FLOWS DURING ELECTROMIGRATION

Non-linear drift velocities have been reported when passing electrical currents through short metal lines. The aluminum drift velocity assum es very small Values or disappears completely when metal lines are sho rt. It was proposed that atoms moving by electromigration cause buildu p of stress gradients (and in some cases concentration gradients) oppo sing their flow. Below a critical length there is no flow and above it , the drift velocity changes linearly with additional current. For alu minum lines with vertical grain boundaries it was suggested that the s tress gradients are sufficient for calculating the back flow. By measu ring the stress gradients in metal lines at or below the critical leng th, the value of the effective charge for aluminum was deduced. This v alue is quite low and does not agree with commonly accepted values. An original experiment is revisited, numerical calculations seem to subs tantiate the low value of the effective charge. Proposals for further experimentation are given. (C) 1998 Acta Metallurgica Inc.