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.