In situ scanning-tunneling-microscopy studies of current driven mass transport in Ag

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Abstract
We present the results of an ultrahigh vacuum (UHV) scanning‐tunneling‐microscopy study of large scale mass transport in current carrying metal films. Scans were taken in situ on a current carrying 2100‐Å‐thick Ag film with current densities ranging from 5.0×104 A/cm2 up to 3.2×105 A/cm2, at which point the sample failed due to a runaway thermal fusing mechanism. Axial UHV inchworms allowed us to obtain data from the same 2‐μm‐square region of the sample throughout the experiment in spite of large temperature‐induced drifts. Calculations of the driving forces demonstrate that the electromigration driving force was dominant for this current range. Significant topographical changes were observed including current aligned grain growth, current induced faceting, and complex mass flow divergences. A model that explains the observed divergences is presented. It is based upon diffusion in the presence of local inhomogeneities of the electric field.

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