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are found to occur along grain boundaries. The strong influence of various microstructural features on void and failure dynamics is observed directly in our experiments. Finally, we show that only 3-dimensional models can describe the microstructural EM-behavior in a realistic way even though the grain structure in the runners is columnar. Practical implications of our findings will be considered. EXPERIMENTAL ASPECTS The experiments were carried out in a JEOL 2000FX transmission electron microscope. A special specimen holder was used which allowed TEM observations to be recorded while simultaneously heating and passing electrical current through the sample. We used the standard current density for accelerated testing of 2x10 6A/cm 2 , and performed the experiment for a variety of temperatures ranging from 200 to 370 0 C. The experiments were done using constant voltage stress conditions. TEM observations were recorded in the conventional way through photographs, as well as through video recording of dynamic processes using a Gatan image intensifier camera and a JVC video cassette recorder (VCR). Samples The samples consisted of 4000 A thick Al(0.5 wt%Cu) patterned into five runners in parallel, all connected to a single bonding pad at each end. The runners were fabricated directly on a thin Si0 2 /SiN (1000 A/ 0 A) ,oo bilayer deposited on a standard Si(000) wafer. After fabrication, the Si under the runners was removed leaving the runners sitting atop an electron-transparent SiU 2 /SiN window allowing for TEM studies. SiN was used on the under-side of the membrane because it stays in tension on the Si wafer, keeping the thin membrane taut even when the Si-substrate is removed to form the window. The lines were 250 pm long and 0.2, 0.3, 0.5, 0.8 and 1.0 ptm in width. Experiments were performed for both the cases of unpassivated and passivated lines, where the passivation consisted of Si0 2 deposited at 350'C to a thickness of 1000 A. On both sides of each Al line on the membrane, were patterned large heat sinks made of the same material as the lines and having the same thickness, but 40 pim in width. Figure 1 shows a schematic of the complete sample. Temperature measurement of the lines and modeling show that such heat sink structures are important for removing the Joule-heating generated by the high current density in the lines. The Joule-heating contribution is included in the temperatures reported below, and is typically 25°C. RESULTS AND DISCUSSION Electromigration causes depletion (voids, thinned-out regions , and failure sites) and accumulation (hillocks, healing, and islands) of Al as well as changes in the grain structure (in some healing processes, or by migration of grain boundaries) by a variety of mechanisms. The dynamics is further complicated by the operation of several different processes simultaneously. Here we give examples of the time-evolution of some of the processes observed [5]. Void Formation Even at the mildest stress conditions studied (current density = 2 x 106A/cm2 , and temperature = 2