Current Flow around Defect Structures

R. Yongsunthon, A. Stanishevsky and E.D. Williams
Materials Research Science and Engineering Center
University of Maryland, College Park

Philip Rous
Department of Physics
University of Maryland, Baltimore County

The metal in electrical wires can actually be displaced by the flow of electrical current through the wires. This electromigration-effect is the principle cause of failure in interconnects, and may dominate the behavior of future nano-scale devices. Mass flow under electromigration is influenced by the tendency of electrical current to “bunch” in the region of corners, bends, cracks or materials defects at the sub-micron size. We have made the first direct observation of current crowding by exploiting the high spatial resolution available using magnetic force microscopy. The MFM signature of a fabricated model defect is shown in the Figure, with the current density obtained by inverting the image. Away from the defect, the current density is uniform, but the current deflected around the defect clearly concentrates at the end of the structure, and there are current “shadows” around the edges of the defect. Imaging current distributions in this way will allow hidden defects to be discovered and characterized, and device characteristics to be quantified in the presence of nanoscale features.
Figure: Left panel - MFM image of 10 micron wide, 0.1 micron thick Au line, with FIB-fabricated diagonal slit-defect. Current applied is 33 mA. Remaining panels: current density determined by deconvolving the instrumental broadening and then performing a numerical inversion using the Biot-Savart Law. The component of the current parallel to line edges is shown in the second panel, the component perpendicular to the edges in the third panel, and the net current in the final panel. The decrease of current density in the “shadow” of the slit is striking, as well as the four-fold increase in current density within one micron of the end of the defect.