Abstract
Thin gold films are potentially important for metallizations in microelectronic devices because of the high activation energy of gold for electrotransport. A high activation energy ensures a longer lifetime of microelectronic devices compared with those in which aluminum metallizations are used. When electromigration is no longer the principal failure mechanism, other failure mechanisms, caused by d.c. stressing, might become important. One possibility is grain boundary grooving. Preliminary studies have shown that grain boundary grooving in thin gold films is prevented by inserting an indium underlay between the gold film and the substrate. The objective of this work was to investigate the mechanisms for the prevention of grain boundary grooving in In/Au composite films by comparing the microstructural evolution of pure gold films with In/Au composite films during isothermal annealing. Microstructures were characterized in terms of grain size, grain size distribution, preferred orientation and surface morphology utilizing transmission electron microscopy (TEM), cross-sectional TEM, scanning electron microscopy and X-ray diffraction. The chemical reactions and the distributions of the phases were monitored by selected area diffraction in TEM, and by Auger electron spectroscopy sputter profiling. It was found that the principal mechanisms that inhibit grain boundary grooving in IN/Au composite films area as follows. 1. (1) Indium underlays modify the microstructure of gold films by randomizing the orientation of the grains, refining the grain size, narrowing the grain size distribution and roughening the surface of the gold films. 2. (2) Indium is redistributed on gold films and forms In 2O 3 on the free surface and within the film during air annealing. 3. (3) The In 2O 3 on the surface “caps” the surface of gold films and limits mass transport during annealing. 4. (4) The In 2O 3 within the gold film, presumably residing at grain boundaries, impedes grain growth by pinning the grain boundary migration.
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