  We report direct observation of the evolution of local temperature
inhomogeneity and the resulting atomic migration in a metal film (Ag on Si)
stressed by a current by using a Scanning Thermal Microscope that allows
simultaneous temperature mapping and topography imaging. The experimental
observation is analyzed using a model based simulation. The experimental
observation and the simulation show that due to current stressing the
temperature of the film becomes significantly inhomogeneous over time (with
local temperature deviating strongly from the mean). This creates local stress
as well as local temperature gradient that lead to mass migration in addition
to the electromigration. We show that the local temperature inhomogeneity
serves as one of the main agents for local atomic migration which leads to
change in film microstructure. The migration leads to damage and eventual
failure as simultaneously monitored by in-situ resistance measurement.
