Phase segregation of Cu in Al–Cu thin films

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Abstract
In the metallization of integrated circuit devices, Cu is often added to Al to prevent the formation of hillocks on the Al films and to inhibit electromigration. In an effort to understand changes in these films which occur during further processing, thin films of an Al–Cu alloy (4 wt.% or 1.7 at.% Cu) sputter deposited on SiO2 were studied by scanning Auger microscopy (SAM). In unheated films, large area depth profiling indicated segregation of the Cu at the SiO2 interface. After heating the film to 500 °C, similar depth profiling showed a uniform distribution of the Cu. However, high spatial resolution (500 Å) SAM observations of the heated films indicated the presence of 1 μm and smaller regions having substantially higher Cu concentrations than the surrounding film. The SAM, combined with argon ion etching, indicated that these regions extend through the depth of the film to the SiO2 substrate. Quantitative AES measured the Cu/Al atomic ratio at about 1/2, suggesting the formation of a theta phase of the Al–Cu alloy. These results imply that a more complicated segregation occurs than is normally measured with large area (>10 μm) Auger spectroscopy.
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