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The
solubility limit of Cu in Al is low as shown by the red solvus line on
the phase diagram.
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The intermetallic compound Al2Cu, the θ-phase,
has a higher atomic percent of copper. In wafer processing, Cu moves in
and out of solution in the alloy, and the θ-phase
precipitates at the grain boundaries.
·
Under operating conditions when electromigration can
occur, the θ-phase
provides a reservoir of Cu atoms that can enter the grain boundaries of
the aluminum and decrease Aluminum electro- migration. This process increases
chip life by decreasing the rate of void formation and fracture due to
aluminum "extrusion." |
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